Add memory pooling for buffers and field maps for improved memory usage and performance

[kainosnoema]

We've seen some high memory and GC pressure in our production deployment. One culprit seems to be func (*Request) execSelections, which can create a lot of buffers that have to grow multiple times in large responses.

One important optimization we can make here is buffer pooling. This PR implements sync.Pool for bytes.Buffer and map[string]*fieldToExec to reduce allocations, memory growth, and GC pressure during execution.

Changes:

• Add internal/exec/pool.go with buffer and field map pooling
• Update Execute(), execSelections(), execList() to use pools
• Apply pooling to subscription handling
• Tests and benchmarks

Isolated Benchmarks

Summary:

• ListQuery: 111µs → 78µs (29% faster), 124KB → 43KB (65% less memory)
• ListWithNestedLists: 428µs → 346µs (19% faster), 637KB → 177KB (72% less memory)
• Concurrent: 219µs → 95µs (56% faster), 659KB → 189KB (71% less memory)

# Without Pooling
BenchmarkMemory_SimpleQuery-12                    115488             10336 ns/op            9925 B/op        126 allocs/op
BenchmarkMemory_ListQuery-12                       10000            111123 ns/op          124602 B/op       2037 allocs/op
BenchmarkMemory_NestedQuery-12                     17557             61133 ns/op           80886 B/op       1074 allocs/op
BenchmarkMemory_ListWithNestedLists-12              2841            428631 ns/op          637248 B/op       9660 allocs/op
BenchmarkMemory_Concurrent-12                       5092            219808 ns/op          659525 B/op      10124 allocs/op
BenchmarkMemory_AllocationsPerOp/Single-12                123319              9433 ns/op            8691 B/op        106 allocs/op
BenchmarkMemory_AllocationsPerOp/List_10-12                23680             42620 ns/op           80833 B/op       1016 allocs/op
BenchmarkMemory_AllocationsPerOp/Nested_Depth3-12          50025             24150 ns/op           37659 B/op        444 allocs/op

# With Pooling
BenchmarkMemory_SimpleQuery-12                    131162              9250 ns/op            7423 B/op        110 allocs/op
BenchmarkMemory_ListQuery-12                       16544             78654 ns/op           43204 B/op       1474 allocs/op
BenchmarkMemory_NestedQuery-12                     23664             59657 ns/op           36693 B/op        810 allocs/op
BenchmarkMemory_ListWithNestedLists-12              3644            346407 ns/op          177613 B/op       6696 allocs/op
BenchmarkMemory_Concurrent-12                      13980             95817 ns/op          189372 B/op       7055 allocs/op
BenchmarkMemory_AllocationsPerOp/Single-12                119922              9149 ns/op            6429 B/op         93 allocs/op
BenchmarkMemory_AllocationsPerOp/List_10-12                39200             30693 ns/op           19197 B/op        655 allocs/op
BenchmarkMemory_AllocationsPerOp/Nested_Depth3-12          58494             20190 ns/op           12544 B/op        309 allocs/op

Real-world Benchmarks (private to our app, includes db i/o)

# Without Pooling
BenchmarkMeQuery-12         	    4756	    708655 ns/op	  104785 B/op	    1377 allocs/op
BenchmarkThreadsQuery-12    	     252	  14639531 ns/op	 5990799 B/op	  101922 allocs/op

# With Pooling
BenchmarkMeQuery-12         	    4786	    693537 ns/op	   91532 B/op	    1351 allocs/op
BenchmarkThreadsQuery-12    	     312	  10358961 ns/op	 5519296 B/op	   98256 allocs/op

[pavelnikolov]

I wonder if these should be configurable through schema options, in case some projects have special requirements...

[pavelnikolov]

We replace this by clear maybe?

Suggested change

	for k := range m {
	delete(m, k)
	}
	clear(m)

[pavelnikolov]

Shouldn't you check this instead of only putting a comment?

[pavelnikolov]

Suggested change

	for i := 0; i < l; i++ {
	for i := range l {

[pavelnikolov]

Suggested change

	func putBuffer(buf *bytes.Buffer) {
	if buf.Cap() > maxBufferCap {
	return
	}
	bufferPool.Put(buf)
	}
	func putBuffer(buf *bytes.Buffer) {
	if buf == nil || buf.Cap() > maxBufferCap {
	return
	}
	bufferPool.Put(buf)
	}

[pavelnikolov]

Suggested change

	func putFieldMap(m map[string]*fieldToExec) {
	if len(m) > maxFieldMapSize {
	return
	}
	func putFieldMap(m map[string]*fieldToExec) {
	if m == nil || len(m) > maxFieldMapSize {
	return
	}