This PR is debugging an issue where schema change propagation (#368)
don't apply properly when there are parallel transactions happening
during an ALTER TABLE statement.

Borrowing from jackc/pglogrepl#59

This PR changes the handling of clientXLogPos to match the one from
Postgres' own `pg_recvlogical`:

- keepalive messages should bump the position too, as they're only sent
(from what I can tell) after any xlogdata message;
- both the WALStart and the ServerWALEnd in logical xlogdata messages
represent the position that should be reported back, and adding the
length of the post-decoding data to it is meaningless;
- relation messages have a position of zero, and in general we should
match the pg_recvlogical behavior of only increasing the local position.

---------

Co-authored-by: Kaushik Iska <[email protected]>

#480)

This PR adds on to the existing work towards syncing schema changes in
CDC mirrors (#368):

1. handles an edge case where a column is dropped and recreated with a
different data type in the same ALTER TABLE statement.
2. Adds quoting, guarding against case-sensitive column names and
reserved identifiers.
3. Fixes type mapping for BigQuery

Additionally, adds some tests for ReplayTableSchemaDelta for Postgres
and Snowflake. Bigquery does not have tests yet but has been manually
tested.