performance/quick-read: access glusterfs_ctx from xlator instead of using glusterfs_get_ctx
performance/quick-read: access glusterfs_ctx from xlator instead of using glusterfs_get_ctx

- since glusterfs_get_ctx gets the global context pointer, there can be
    problems in a multithreaded application running on libglusterfsclient
    doing multiple glusterfs_inits. Hence use context specific to the
    current xlator tree stored in each xlator object.

Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 240 (segmentation fault in qr_readv)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=240

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster