Introduce explicit policy for within-dialog SST support:
- whenever remote side (call leg) doesn't explicitly declare
it supports timers (with `Supported: timer`), disable all
previously set timers for this particular leg
- this touches request originator, as well as answerer
- requests: only UPDATE and INVITE methods are affected
- answers: only positive answers are affected
- by default this behavior is always disabled if not enabled
explicitly in the profile's config with `sst_strict_mode`
- sst_strict_mode takes either `yes` or `no`
P.S.: the `sst_strict_mode` must not be initialized to true
by default, because RFC 4028 explicitly tells to support
the operation even when only one UA understands session timers.
So we have to be quite accurate and not enforce the strict mode.
Change-Id: Ie6647eb061a73dc052367dbd291589198487093c
Donat Zenichev