Complete documentation added for the Call Center module

(cherry picked from commit 8f0a1508729276b8b8057e215e20316039f07079)
call_center: add column info when loading calls from db

(cherry picked from commit 997ccd335b1ec93397a0d86018ff8543a1cf71ae)
call_center: add last_call_end column

(cherry picked from commit 4b366c5b3866f1541e073c8799022fbe74e53392)
call_center: add cc_calls table

(cherry picked from commit 2f6eb09c19e01043d167fe168ba45ca3d6163e0e)
call_center: fix default scenario readme

(cherry picked from commit a5f6e10ae6e780e7569bca7099de8f3b0be57bcd)
b2b_logic: do not call the DESTROY callback at shutdown

(cherry picked from commit 4ddf1b37cc27a918ba209c06f3cb65f5e3d4342a)
call_center: initialize ret variable

(cherry picked from commit e086595b812587ba15895bbf6b7fea9e8cc4ebd7)
b2b: restore callbacks mask after reload

(cherry picked from commit fc3ca077ead8b85098f77e48ef8bb6b193512847)