- simplify general implementation - avoid deadlock due to a process trying to acquire same lock twice

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#### Description <!-- Describe your changes in detail --> On a timer, checking the availability of the master db used for writing and setting a status variable accordingly. In case the mdb_availability_control option is set, this status variable is consulted with no synchronization by each process when trying to do a write to the master db. As error mitigation, in case the per process handle associated with master db is NULL, p_usrlock recreates it. In case the per process handle is not NULL, Kamailio will try to recreate a broken connection by itself. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/2399

-- Commit Summary --

* p_usrloc: reworking mdb_availability_control

-- File Changes --

M src/modules/p_usrloc/p_usrloc_mod.c (23) M src/modules/p_usrloc/p_usrloc_mod.h (6) M src/modules/p_usrloc/ul_db.c (70) M src/modules/p_usrloc/ul_db_watch.c (15) M src/modules/p_usrloc/ul_db_watch.h (2)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2399.patch https://github.com/kamailio/kamailio/pull/2399.diff