Summary:
PG 15 added a new interrupt ProcSignalBarrierPending which is
set to true after certain queries, ProcessProcSignalBarrier()
must be called, otherwise ASH collector is stuck, as seen by
the PgWaitEventAuxTest.NewDatabaseRPCs test.

This test was created before PG 15 merge, and skipped
when D38309 / 72fc4677c578973e21b31a513a03fe948b2ed8fc
was landed with PG 15 changes.

Summary of changes -
- Remove signal handlers defined in yb_ash.h
- Add signal handlers for SIGINT and SIGQUIT too
- Use HandleMainLoopInterrupts to handle signals

Jira: DB-13621

Test Plan:
Jenkins

./yb_build.sh --cxx-test pg_ash-test --gtest_filter *NewDatabaseRPCs

Reviewers: jason

Reviewed By: jason

Subscribers: amitanand, hbhanawat, yql

Differential Revision: https://phorge.dev.yugabyte.com/D39533