Fix duplicate key errors on resuming continued txn (#555)

This commit addresses and resolves the issue of duplicate key errors when resuming partially executed transactions (continuedTxn) in pgcopydb. We have reintroduced the transaction metadata file, which is essential for identifying the commitLSN of a partial transaction.

Unlike our previous approach, which led to a deadlock between the transform and apply phases, this update brings a more efficient process. Now, the apply phase creates metadata for any partial (continued) transactions during the commit. This metadata is then used to accurately skip the already applied partial transaction if a resume is needed.

This fix is crucial, particularly for tables with unique constraints, where executing the same continued transaction twice previously resulted in duplicate key errors. With this update, pgcopydb ensures smooth and error-free handling of transaction resumes.

Signed-off-by: Arunprasad Rajkumar <ar.arunprasad@gmail.com>