Some code reorganization

Some code reorganization

While working on the pcp_stop_pgpool command to implement cluster mode,
I realized that we have a lot of clutter in a few source files. Especially
pool.h header file. So I also did some code reorganization and added a
bunch of new includefiles in the hope to reduce the size of pool.h and
also get rid of too many global functions.

Along the way, I found out there are few other files that need a little
shredding as well, src/main/pgpool_main.c and src/protocol/child.c are at
the top of this wanted to be trimmed list. So as part of this commit.

I have taken a few things related to internal commands
(interface between the child processes can and Pgpool-II main process)
from pgpool_main.c file and moved them into new "src/main/pool_internal_comms.c"

Similarly, src/protocol/child.c had and still has so many functions that do
not fit with the personality of child.c file. So I have moved some of the
stuff related to DB functions info src/protocol/pool_pg_utils.c file.

Having done that I think this is still not enough and we may require another
round ( if someoneis willing to work on it) of a source file reorganization.