FreeSWITCH-FS-6798

allow enter and exit sounds to interrupt the MOH in a wait_mod conference

This patch does the following:
* only starts MOH if no other file (sync or async) is currently playing
* adds a variable "conference_permanent_wait_mod_moh" that controls the
  behavior of how the enter and exit sounds interact with the MOH when
  wait_mod is set. When the variable is set, the MOH keeps playing and
  the enter and exit sounds are mixed with the MOH. When the variable
  is unset, then any playing MOH is first stopped, then the enter or
  exit sound is played and the MOH is started again.

This functionality is useful in case the enter and exit sounds are
used to announce the name of the caller, who is joining or leaving a
conference.

FS-5159 #resolve