LaxConnPool: Deallocate pool entry upon discarding expired connection

[rschmitt]

This will be backported to the 5.4.x branch.

[arturobernalg]

Good catch—thanks for fixing the allocation leak

[rschmitt]

By the way, it appears that STRICT (the default) and OFFLOCK do not exhibit this issue, but I wouldn't mind if someone wanted to double-check. This is the third LaxConnPool bug and the fourth pooling bug overall I've found in the last month, so we should think about implementing some sort of souped-up testing strategy like generative contract testing (i.e. run a common set of fuzz tests on all three implementations and check appropriate invariants).