Change the signaling between startup process and postmaster. Startup process

Change the signaling between startup process and post Startup process
now tells postmaster about the start, consistent point, and completion of
WAL redo, using signals. It now returns with exit code 0 when WAL replay is
aborted with SIGTERM. Background writer is launched as soon as we start WAL
redo, so that it can perform restartpoints before reaching consistent state.
Postmaster handles unexpected death of the startup process gracefully. In
case of a fast shutdown, bgwriter performs a "shutdown restartpoint", similar
to a shutdown checkpoint.