cpp: Fixed successive batch updates on same top level key [ATTENTION]

Read access in locked payloads is performed on queued operations
before searching the old database. Threads other than the locking
thread consistently read the old information.

On POSIX, default Lock now also uses boost::thread (instead of pthread)
pthread linkage is still present for legacy wrapping.

ATTENTION: unlocked payload writing is (by design) not thread-safe
(won't crash but data might be inconsistent). Please use Locker armors
around all code that might modify a payload in more than one thread.
---> #1: Always use Locker in IU handlers that write to existing IUs!!