pgsql: First-draft release notes for 15.4.

First-draft release notes for 15.4. As usual, the release notes for other branches will be made by cutting these down, but put them up for community review first. Branch -- REL_15_STABLE Details --- https://git.postgresql.org/pg/commitdiff/c6344d7686f3e3c8243c2c6771996cfc63e71eae Modifi

pgsql: Refactor to split Apply and Tablesync Workers code.

Refactor to split Apply and Tablesync Workers code. Both apply and tablesync workers were using ApplyWorkerMain() as entry point. As the name implies, ApplyWorkerMain() should be considered as the main function for apply workers. Tablesync worker's path was hidden and does not have enough in commo