Silence some clang warnings
Plug leak in 4d02c741b120786df1b87ee9ed49c1d3f9bc7522

Free writer txn on env_close
ITS#7942
ITS#7942 tweak previous commit

Only free when cleaning up operation controls, not response controls.
(This only makes a difference on search requests, which may have
multiple responses with the same set of controls.)
ITS#7943 reinit txn flags

When reusing the main write txn
ITS#7971 LMDB: clarification in mdb_txn_renew0().
ITS#7970 LMDB: Critical Heisenbug

Inconsistent reading & SIGSEGV due to the race condition.
ITS#7969 LMDB: volatile & __synchronize().

Globally shared fields of meta-data are not 'volatile'.
 TRIVIA - lmdb: clean testdb-dir while "make test".
 EXTENSION - lmdb: more usefull info from mdb_stat tool.
 CHANGE - lmdb-backend: checkpoint-interval in seconds instead of minutes.
 FEATURE - lmdb: implementation of "checkpoint kbytes".

 Force flush when volume of the changes reached a configurable threshold.
 FEATURE - lmdb-backend: support for "checkpoint kbytes" config-option.
 BUGFIX - lmdb: properly sync meta-pages in mdb_sync_env().

 Meta-pages may be updated during data-syncing in mdb_sync_env(),
 in this case database would be inconsistent.

 Check-and-retry if lead txn-id changed during flushing data in mdb_sync_env().
 FEATURE - lmdb: MDB_LIFORECLAIM & MDB_COALESCE modes.

 Reclaim FreeDB in LIFO order - this is a main feature.
 Also aim to coalesce small FreeDFB records.
 FEATURE - lmdb-backend: support config for 'lifo' and 'coalesce' envflags.
 BUGFIX - lmdb: lock meta-pages in writemap-mode to avoid unexpected write,
           before the data pages would be synchronized.

 Without locking the meta-pages may be writen by OS before other data,
 in this case database would be inconsistent.
 ITS#7965 mdb_dn2id: heap corruption due to returning a reference to the local variable.
 EXTENSION - lmdb-backend: stack backtrace on SIGSEGV and friends (optional).
Silence compiler warning by adding explicit return 0 to ppolicy_db_destroy