tmx: backup and restore existing T and branch pointers for t_cancel_callid()

- reported and initial patch by Guillaume Bour

(cherry picked from commit 60372d613544af1f8f06122eb77fa2a9636a6c3a)
(cherry picked from commit f7ed69d75746dc55e5a48a7a18602124e73cd3cd)
tm: exported set_t() and get_t_branch() viam tm api

- allows to backup and restore pointers to T and branch

(cherry picked from commit 777dd5e28ff6b4fcc3b1c44e841eb415c974075f)
(cherry picked from commit 09fb0aae62761ce4f251ab22c6ab9cde55e2c663)
core: added function to check if counters have been initialized

(cherry picked from commit 8161152b320f3b430523ea08e86a56f79405c2f4)
(cherry picked from commit a165fa668378da5c312a0c268aaf158ff583127f)
core: don't increment failed dns query counter for reverse dns lookup at startup

- it can happen before counters are intialized, thus check that before
- reported by Hugh Waite, FS#375

(cherry picked from commit 89c6d73dd51eb9b96f50d3f04219612fa7725224)
(cherry picked from commit a3169d02d7527110941fab8de25ba75aae759ce1)
acc: added note about MySQL InnoDB not supporting INSERT DELAYED

- reported by Daniel Tryba

(cherry picked from commit d26eff22c8a543c63a40cd7d91bb3c70f04aabc6)
(cherry picked from commit 4ac82cdf92ae6c0e2ff9c1238bfe28cfcefea99c)
modules: bulk regeneration of README files

- some modules had xml files updated, but no refresh for README files