chan_pjsip: Add support for passing hold and unhold requests through.

This change adds an option, moh_passthrough, that when enabled will pass
hold and unhold requests through using a SIP re-invite. When placing on
hold a re-invite with sendonly will be sent and when taking off hold a
re-invite with sendrecv will be sent. This allows remote servers to handle
the musiconhold instead of the local Asterisk instance being responsible.

Review: https://reviewboard.asterisk.org/r/4103/

Change-Id: Ib6294e906e577e1a4245cb1f058d3976ff484c52

contrib/ast-db-manage/config/versions/339e1dfa644d_add_moh_passthrough_option_to_pjsip.py

@@ -0,0 +1,30 @@
+"""Add moh_passthrough option to pjsip
+
+Revision ID: 339e1dfa644d
+Revises: 3a094a18e75b
+Create Date: 2014-10-21 14:55:34.197448
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '339e1dfa644d'
+down_revision = '3a094a18e75b'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import ENUM
+
+YESNO_NAME = 'yesno_values'
+YESNO_VALUES = ['yes', 'no']
+
+def upgrade():
+    ############################# Enums ##############################
+
+    # yesno_values have already been created, so use postgres enum object
+    # type to get around "already created" issue - works okay with mysql
+    yesno_values = ENUM(*YESNO_VALUES, name=YESNO_NAME, create_type=False)
+
+    op.add_column('ps_endpoints', sa.Column('moh_passthrough', yesno_values))
+
+def downgrade():
+    op.drop_column('ps_endpoints', 'moh_passthrough')