Add option to bypass dropped column erasure.

Signed-off-by: Jason Volk <jason@zemos.net>

tuwunel-example.toml

@@ -1080,6 +1080,18 @@
 #
 #rocksdb_stats_level = 1
 
+# Erases data no longer reachable in the current schema. The developers
+# expect this to be set to true which simplifies the schema and prevents
+# accumulation of old schemas remaining in the codebase forever. If this
+# is set to false, old columns which are not described in the current
+# schema will be ignored rather than erased, leaking their space.
+#
+# This can be set to false when moving between versions in ways which are
+# not recommended or otherwise forbidden, or for diagnostic and
+# development purposes; requiring preservation across such movements.
+#
+#rocksdb_drop_missing_columns = true
+
 # This is a password that can be configured that will let you login to the
 # server bot account (currently `@conduit`) for emergency troubleshooting
 # purposes such as recovering/recreating your admin room, or inviting