mempool: release lock during app connection flush (#8986)
mempool: release lock during app connection flush (#8986)

A manual backport of #8984.

This case is symmetric to what we did for CheckTx calls, where we release the
mempool mutex to ensure callbacks can fire during call setup.  We also need
this behaviour for application flush, for the same reason: The caller holds the
lock by contract from the Mempool interface.
tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint