This change makes SynRcvdCountThreshold and the global synRcvdCount into a stack
configurable value. This is required because in cases like mod_proxy which
create multiple Stack instances the count will be a global value that impacts
all Stack instances.
Further the tests relied on modifying the global threshold to simulate tests
where we want to verify SYN cookie based behaviour. This lead to data races due
to the global being modified/read without locks or atomics.
PiperOrigin-RevId: 306947723
0eda0104a5