ITS#5452, ITS#8012: fix syncops list iteration and detaching on freed;

Detaching a syncops record from op-list conditionally,
only when it was freed by syncprov_free_syncop.

The syncprov_drop_psearch() and syncprov_drop_psearch() now
returns a flag, which is nonzero if the given syncops was freed.
slapd: ITS#7974 dreamcatcher feature.
lmdb: ITS#7974 oomkiller feature.
slapd: ITS#7974 oomkiller feature.
lmdb: MDB_LIFORECLAIM & MDB_COALESCE modes.

Reclaim FreeDB in LIFO order - this is a main feature.
Also aim to coalesce small FreeDFB records.
lmdb-backend: support config for 'lifo' and 'coalesce' envflags.
back-mdb: stack backtrace on SIGSEGV and friends.
back-mdb: stack backtrace enabled only if symbols available.
syncrepl: ANY-logging for delete-ops.
syncrepl: ANY-logging for REFRESH.