A new function to wait for the backend exit after termination

Hi,

Currently pg_terminate_backend(), sends SIGTERM to the backend process but
doesn't ensure it's exit. There are chances that backends still are
running(even after pg_terminate_backend() is called) until the interrupts
are processed(using ProcessInterrupts()). This could cause problems
especially in testing, for instance in a sql file right after
pg_terminate_backend(), if any test case depends on the backend's
non-existence[1], but the backend is not terminated. As discussed in [1],
we have wait_pid()(see regress.c and sql/dblink.sql), but it's not usable
across the system. In [1], we thought it would be better to have functions
ensuring the backend's exit on the similar lines of pg_terminate_backend().

I propose to have two functions:

1. pg_terminate_backend_and_wait() -- which sends SIGTERM to the backend
and wait's until it's exit.
2. pg_wait_backend() -- which waits for a given backend process. Note that
this function has to be used carefully after pg_terminate_backend(), if
used on a backend that's not ternmited it simply keeps waiting in a loop.

Attaching a WIP patch herewith.

Thoughts?

[1]
https://www.postgresql.org/message-id/flat/f31cc4da-a7ea-677f-cf64-a2f9db854bf5%40oss.nttdata.com

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com