Refactor ProcessRecoveryConflictInterrupt for readability

Refactor ProcessRecoveryConflictInterrupt for readability

Two changes here:

1. Introduce a separate RECOVERY_CONFLICT_BUFFERPIN_DEADLOCK flag to
indicate a suspected deadlock that involves a buffer pin. Previously
the startup process used the same flag for a deadlock involving just
regular locks, and to check for deadlocks involving the buffer
pin. The cases are handled separately in the startup process, but the
receiving backend had to deduce which one it was based on
HoldingBufferPinThatDelaysRecovery(). With a separate flag, the
receiver doesn't need to guess.

2. Rewrite the ProcessRecoveryConflictInterrupt() function to not rely
on fallthrough through the switch-statement. That was difficult to
read.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/4cc13ba1-4248-4884-b6ba-4805349e7f39@iki.fi