Eliminate replication protocol error related to IDENTIFY_SYSTEM.

Eliminate replication protocol error related to IDENTIFY_SYSTEM.

The requirement that IDENTIFY_SYSTEM be run before START_REPLICATION
was both undocumented and unnecessary. Remove the error and ensure
that ThisTimeLineID is initialized in START_REPLICATION.

Elect not to backport because this requirement was expected behavior
(even if inconsistently enforced), and is not likely to cause any
major problem.

Author: Jeff Davis
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/de4bbf05b7cd94227841c433ea6ff71d2130c713.camel%40j-davis.com