lmdb-safe: Improve the scalability of transaction maps

lmdb-safe: Improve the scalability of transaction maps

This commit improves the scalability of the transaction maps by
moving from a `mutex` to a `shared_mutex` and making the stored value
atomic. This allows accessing the maps concurrently from different threads
as long as the entry for the thread exists, and we fall back to
taking a full lock otherwise.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>
(cherry picked from commit c340aa91bf37d8105d2b2390eecbadfca88c1d27)