Skip empty transactions for logical replication.

Skip empty transactions for logical replication.

The current logical replication behavior is to send every transaction to
subscriber even if the transaction is empty. This can happen because
transaction doesn't contain changes from the selected publications or all
the changes got filtered. It is a waste of CPU cycles and network
bandwidth to build/transmit these empty transactions.

This patch addresses the above problem by postponing the BEGIN message
until the first change is sent. While processing a COMMIT message, if
there was no other change for that transaction, do not send the COMMIT
message. This allows us to skip sending BEGIN/COMMIT messages for empty
transactions.

When skipping empty transactions in synchronous replication mode, we send
a keepalive message to avoid delaying such transactions.

Author: Ajin Cherian, Hou Zhijie, Euler Taveira
Reviewed-by: Peter Smith, Takamichi Osumi, Shi Yu, Masahiko Sawada, Greg Nancarrow, Vignesh C, Amit Kapila
Discussion: https://postgr.es/m/CAMkU=1yohp9-dv48FLoSPrMqYEyyS5ZWkaZGD41RJr10xiNo_Q@mail.gmail.com