res_pjsip: Add rtp_keepalive endpoint option.

This adds an "rtp_keepalive" option for PJSIP endpoints. Similar to the
chan_sip option, this specifies an interval, in seconds, at which we
will send RTP comfort noise frames. This can be useful for keeping RTP
sessions alive as well as keeping NAT associations alive during lulls.

ASTERISK-25242 #close
Reported by Mark Michelson

Change-Id: I3b9903d99e35fe5d0b53ecc46df82c750776bc8d

contrib/ast-db-manage/config/versions/498357a710ae_add_rtp_keepalive.py

@@ -0,0 +1,22 @@
+"""Add RTP keepalive
+
+Revision ID: 498357a710ae
+Revises: 28b8e71e541f
+Create Date: 2015-07-10 16:42:12.244421
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '498357a710ae'
+down_revision = '28b8e71e541f'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column('ps_endpoints', sa.Column('rtp_keepalive', sa.Integer))
+
+
+def downgrade():
+    op.drop_column('ps_endpoints', 'rtp_keepalive')