res_pjsip_pubsub: Prune subs with reliable transports at startup

In an earlier release, inbound registrations on a reliable transport
were pruned on Asterisk restart since the TCP connection would have
been torn down and become unusable when Asterisk stopped.  This same
process is now also applied to inbound subscriptions.

Also fixed issues in res_pjsip_registrar where it wasn't handling the
monitoring correctly when multiple registrations came in over the same
transport.

To accomplish this, the pjsip_transport_event feature needed to
be refactored to allow multiple monitors (multiple subcriptions or
registrations from the same endpoint) to exist on the same transport.
Since this changed the API, any external modules that may have used the
transport monitor feature (highly unlikey) will need to be changed.

ASTERISK-27612
Reported by: Ross Beer

Change-Id: Iee87cf4eb9b7b2b93d5739a72af52d6ca8fbbe36

contrib/ast-db-manage/config/versions/d3e4284f8707_add_prune_on_boot_to_ps_subscription_.py

@@ -0,0 +1,33 @@
+"""add prune_on_boot to ps_subscription_persistence
+
+Revision ID: d3e4284f8707
+Revises: 52798ad97bdf
+Create Date: 2018-01-28 17:45:36.218123
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'd3e4284f8707'
+down_revision = '52798ad97bdf'
+
+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_subscription_persistence', sa.Column('prune_on_boot', yesno_values))
+
+def downgrade():
+    if op.get_context().bind.dialect.name == 'mssql':
+        op.drop_constraint('ck_ps_subscription_persistence_prune_on_boot_yesno_values','ps_subscription_persistence')
+    op.drop_column('ps_subscription_persistence', 'prune_on_boot')