pg_prewarm: make autoprewarm leader use standard SIGHUP and SIGTERM handlers.

pg_prewarm: make autoprewarm leader use standard SIGHUP and SIGTERM handlers.

Commit 1e53fe0e70 changed background processes so that they use
standard SIGHUP handler. Like that, this commit makes autoprewarm leader
process also use standard SIGHUP and SIGTERM handlers, to simplify the code.

Author: Bharath Rupireddy
Reviewed-by: Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/CALj2ACXPorUqePswDtOeM_s82v9RW32E1fYmOPZ5NuE+TWKj_A@mail.gmail.com