feat: add PackageBase.MetaModifiedTS column

Since we cannot depend on InnoDB features across all mysql
installations, we are instead using this new column to track
metadata changes for a particular package base.

This will allow us to fetch less data when producing metadata
archives for end-users, and it does not depend on installation
details aside from the schema/migrations being applied.

New columns:
- `PackageBase`.`MetaModifiedTS`

Signed-off-by: Kevin Morris <kevr@0cost.org>

migrations/versions/42584a60e7a7_add_metamodifiedts_to_packagebase.py

@@ -0,0 +1,31 @@
+"""add MetaModifiedTS to PackageBase
+
+Revision ID: 42584a60e7a7
+Revises: d64e5571bc8d
+Create Date: 2022-09-06 12:37:49.460344
+
+"""
+from alembic import op
+from sqlalchemy.exc import OperationalError
+
+from aurweb.models import PackageBase
+
+# revision identifiers, used by Alembic.
+revision = "42584a60e7a7"
+down_revision = "d64e5571bc8d"
+branch_labels = None
+depends_on = None
+
+table = PackageBase.__tablename__
+
+
+def upgrade():
+    try:
+        op.add_column(table, PackageBase.__table__.c.MetaModifiedTS)
+    except OperationalError as e:
+        print(e)
+        print("MetaModifiedTS column already exists")
+
+
+def downgrade():
+    op.remove_column(table, "MetaModifiedTS")