Remove uses of shared_ptr::use_count == 1 ?

[kgpai]

Bug description

Recently MacOS SDK deprecated use of shared_ptr::unique (see #11318 ). A workaround is to use shared_ptr::use_count() ==1 , which is what was used underneath by unique(). The problem is that use_count() is unreliable and not meant to be used when the shared_ptr is used across threads , see : https://stackoverflow.com/q/77286007

A lot of places use use_count() == 1 to decide if its safe to mutate said shared_ptr and all of these use cases assume that the Vector is not shared across multiple threads or is synchronized by some other means.

System information

N/a

Relevant logs

No response

[kgpai]

cc: @kevinwilfong @Yuhta @czentgr

[Yuhta]

We assume each thread using vector should keep a copy of shared_ptr. With this assumption hold the current usages are safe.

We should not expect client code to hold this assumption though, so external code outside Velox should be advised to use a mutex to synchronize the multi-threaded usage of vectors.