Improve error message when standby does accept connections.

Improve error message when standby does accept connections.

Even after reaching the minimum recovery point, if there are long-lived
write transactions with 64 subtransactions on the primary, the recovery
snapshot may not yet be ready for hot standby, delaying read-only
connections on the standby. Previously, when read-only connections were
not accepted due to this condition, the following error message was logged:

    FATAL:  the database system is not yet accepting connections
    DETAIL:  Consistent recovery state has not been yet reached.

This DETAIL message was misleading because the following message was
already logged in this case:

    LOG:  consistent recovery state reached

This contradiction, i.e., indicating that the recovery state was consistent
while also stating it wasn’t, caused confusion.

This commit improves the error message to better reflect the actual state:

    FATAL: the database system is not yet accepting connections
    DETAIL: Recovery snapshot is not yet ready for hot standby.
    HINT: To enable hot standby, close write transactions with more than 64 subtransactions on the primary server.

To implement this, the commit introduces a new postmaster signal,
PMSIGNAL_RECOVERY_CONSISTENT. When the startup process reaches
a consistent recovery state, it sends this signal to the postmaster,
allowing it to correctly recognize that state.

Since this is not a clear bug, the change is applied only to the master
branch and is not back-patched.

Author: Atsushi Torikoshi <torikoshia@oss.nttdata.com>
Co-authored-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Yugo Nagata <nagata@sraoss.co.jp>
Discussion: https://postgr.es/m/02db8cd8e1f527a8b999b94a4bee3165@oss.nttdata.com