Improve memory handling across SQL-callable backup functions

Since pg_backup_start() and pg_backup_stop() exist, the tablespace map
data and the backup state data (backup_label string until 7d70809) have
been allocated in the TopMemoryContext.  This approach would cause
memory leaks in the session calling these functions if failures happen
before pg_backup_stop() ends, leaking more memory on repeated failures.
Both things need little memory so that would not be really noticeable
for most users, except perhaps connection poolers with long-lived
connections able to trigger backup failures with these functions.

This commit improves the logic in this area by not allocating anymore
the backup-related data that needs to travel across the SQL-callable
backup functions in TopMemoryContext, by using instead a dedicated
memory context child of TopMemoryContext.  The memory context is created
in pg_backup_start() and deleted when finishing pg_backup_stop().  In
the event of an in-flight failure, this memory context gets reset in the
follow-up pg_backup_start() call, so as we are sure that only one run
worth of data is leaked at any time.  Some cleanup was already done for
the backup data on a follow-up call of pg_backup_start(), but using a
memory context makes the whole simpler.

BASE_BACKUP commands are executed in isolation, relying on the memory
context created for replication commands, hence these do not need such
an extra logic.

Author: Bharath Rupireddy
Reviewed-by: Robert Haas, Alvaro Herrera, Cary Huang, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACXqvfKF2B0beQ=aJMdWnpNohmBPsRg=EDQj_6y1t2O8mQ@mail.gmail.com
