Various fixes related to instance tags:
* get_instag(): return meta OTRL_INSTAG_BEST instead of creating new instag
* otrl_context_find_v3(): always use OTRL_INSTAG_BEST
* OtrInternal ctor: use correct file name to load instance
Fixed libotr flags: OTRL_POLICY_REQUIRE_ENCRYPTION -> OTRL_POLICY_ALWAYS