lib ldb: lmdb clear stale readers on write txn start

lib ldb: lmdb clear stale readers on write txn start

In use process failures and Bind9 shut downs leave stale entries in the
lmdb reader table.  This can result in lmdb filling it's database file, as
the free list can not be reclaimed due to the stale reader.

In this fix we call mdb_reader_check at the start of each transaction,
to free any stale readers.  As the default maximum number of readers is
127, this should not impact on performance to any great extent.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14330

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Mar 31 01:26:07 UTC 2020 on sn-devel-184