SedRegex: Cache ircdb.checkIgnored during a request

[progval]

Benchmarking shows a 2 to 4 seconds improvement for all multiprocessing methods:

method	cached	target check	far	middle	last
fork	no	before checkIgnored	17.0	17.1	13.3
fork	yes	before checkIgnored	12.6	13.5	11.0
fork	no	after checkIgnored	16.7	17.1	13.4
fork	yes	after checkIgnored	12.6	13.4	11.2
list fork	no	before checkIgnored
list fork	yes	before checkIgnored	10.2	10.8	10.7
list fork	no	after checkIgnored	14.7	15.3	14.9
list fork	yes	after checkIgnored	10.2	10.7	10.5
forkserver	no	before checkIgnored	171.9	171.9	172.3
forkserver	yes	before checkIgnored	168.5	167.8	169.6
forkserver	no	after checkIgnored
forkserver	yes	after checkIgnored

    def testPerfLast(self):
        for i in range(10000):
            self.feedMsg(f'xxx {i} yyy')
        self.assertResponse('ping', 'pong')

        before = time.time()
        for _ in range(1000):
            self.feedMsg('s/xxx 9999 yyy/foo/')  # no ^ or $, so it matches itself
        after = time.time()
        print(after - before)
        assert False

    def testPerfMiddle(self):
        for i in range(10000):
            self.feedMsg(f'xxx {i} yyy')
        self.assertResponse('ping', 'pong')

        before = time.time()
        for _ in range(1000):
            self.feedMsg('s/^xxx 1234 yyy$/foo/')  # I meant to use 9876 here. so it's actually the same test as testPerfFar. too bad.
        after = time.time()
        print(after - before)
        assert False

    def testPerfFar(self):
        for i in range(10000):
            self.feedMsg(f'xxx {i} yyy')
        self.assertResponse('ping', 'pong')

        before = time.time()
        for _ in range(1000):
            self.feedMsg('s/^xxx 1234 yyy$/foo/')
        after = time.time()
        print(after - before)
        assert False

"list fork" means I used fork with this patch:

diff --git a/plugins/SedRegex/plugin.py b/plugins/SedRegex/plugin.py
@@ -242,8 +246,7 @@ def doPrivmsg(self, irc, msg):
         if self.registryValue('boldReplacementText', msg.channel, irc.network):
             replacement = ircutils.bold(replacement)
         try:
-            if isinstance(world.SUPYPROCESS_MULTIPROCESSING_CONTEXT,
-                          multiprocessing.context.ForkContext):
+            if False:
                 # global state is shared with child processes, so the child
                 # process has access to history and can lazily filter it
                 message = process(self._replacer_process, irc, msg,