LU-14291 build: use tgt_pool for lov layer

New general code was created for target pool handling. We can
use this new code with the lov layer. Place this tgt_pool.c in
the obdclass instead of having a special target directory just to
build this code for the client.

Change-Id: I05542c1d654d79647f5e0853bb1d587ff265fdf9
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/39683
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>