parser: make the parser thread safe.

The volfile parser thread safe by guarding the parsing phase
in a mutex. Thread safety becomes a problem when there are multiple
glfs_t objects created by gfapi and all of them potentially parse
the respective volfiles at the same time.

Change-Id: I4376019c4956994b72397ab36e6ac3ce849797ec
BUG: 1004519
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5872
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>