Crash on multiple connections/disconnections

[romansimakov]

It's easily reproducable by attached python script on default build of master (SS).
nbak-test.zip

It looks like reallocated mem. And it might happen the second cache_writer for the same dbb.

[AlexPeshkoff]

The reason of a bug is ability to start more than one cache_writer thread simultaneously due to races on bcb_flags in CCH_init2.

[pavel-zotov]

Could not reproduce on 5.0.0.1182 SS (Windows). Sent report to Alex and Roman.