res_pjsip_registrar: Remove unavailable contacts if exceeds max_contacts

The behavior of max_contacts and remove_existing are connected.  If
remove_existing is enabled, the soonest expiring contacts are removed.
This may occur when there is an unavailable contact.  Similarly,
when remove_existing is not enabled, registrations from good
endpoints are rejected in favor of retaining unavailable contacts.

This commit adds a new AOR option remove_unavailable, and the effect
of this setting will depend on remove_existing.  If remove_existing
is set to no, we will still remove unavailable contacts when they
exceed max_contacts, if there are any. If remove_existing is set to
yes, we will prioritize the removal of unavailable contacts before
those that are expiring soonest.

ASTERISK-29525

Change-Id: Ia2711b08f2b4d1177411b1be23e970d7fdff5784

contrib/ast-db-manage/config/versions/f56d79a9f337_pjsip_create_remove_unavailable.py

@@ -0,0 +1,30 @@
+"""pjsip create remove_unavailable
+
+Revision ID: f56d79a9f337
+Revises: c20d6e3992f4
+Create Date: 2021-07-28 02:09:11.082061
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'f56d79a9f337'
+down_revision = 'c20d6e3992f4'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import ENUM
+
+AST_BOOL_NAME = 'ast_bool_values'
+AST_BOOL_VALUES = [ '0', '1',
+                    'off', 'on',
+                    'false', 'true',
+                    'no', 'yes' ]
+
+def upgrade():
+    ast_bool_values = ENUM(*AST_BOOL_VALUES, name=AST_BOOL_NAME, create_type=False)
+    op.add_column('ps_aors', sa.Column('remove_unavailable', ast_bool_values))
+
+def downgrade():
+    op.drop_column('ps_aors', 'remove_unavailable')
+    pass
+