pgbouncer - pgbouncer: Reserve pool implementation.

Log Message:
-----------
Reserve pool implementation.

Allow to increased pool size in case main pool size
cannot serve clients anymore.

This allows to continue use of small pool_size, which
smooths small peaks, while also handling large peaks.

(Eg, one bouncer server down, others need to handle load.)

Modified Files:
--------------
pgbouncer/doc:
config.txt (r1.17 -> r1.18)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/doc/config.txt.diff?r1=1.17&r2=1.18)
pgbouncer/etc:
pgbouncer.ini (r1.7 -> r1.8)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/etc/pgbouncer.ini.diff?r1=1.7&r2=1.8)
pgbouncer/include:
bouncer.h (r1.21 -> r1.22)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/include/bouncer.h.diff?r1=1.21&r2=1.22)
pgbouncer/src:
admin.c (r1.38 -> r1.39)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/admin.c.diff?r1=1.38&r2=1.39)
janitor.c (r1.31 -> r1.32)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/janitor.c.diff?r1=1.31&r2=1.32)
loader.c (r1.19 -> r1.20)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/loader.c.diff?r1=1.19&r2=1.20)
main.c (r1.57 -> r1.58)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/main.c.diff?r1=1.57&r2=1.58)
objects.c (r1.50 -> r1.51)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/objects.c.diff?r1=1.50&r2=1.51)