res_pjsip_exten_state,res_pjsip_mwi: Allow unload on shutdown

res_pjsip_exten_state,res_pjsip_mwi: Allow unload on shutdown

Commit f66f77f last year prevents the res_pjsip_exten_state and
res_pjsip_mwi modules from unloading due to possible pjproject
asserts if the modules are reloaded. A side effect of the
implementation is that the taskprocessors these modules use aren't
being released. When asterisk is doing a graceful shutdown, it
waits AST_TASKPROCESSOR_SHUTDOWN_MAX_WAIT seconds for all
taskprocessors to stop but since those 2 modules don't release
theirs, the shutdown hangs for that amount of time.

This change allows the modules to be unloaded and their resources to
be released when ast_shutdown_final is true.

Resolves: #379