gfapi: optimistic resolution with ESTALE detection

Enhance the path resolver to be optimistic. Instead of performing
a wasteful ->lookup() fop for every component in every API call,
build in speculation logic to consider the previous resolution
result temporarily, and if it results in ESTALE either later in
the path resolution or in the FOP, then retry resolution with
->lookup() at every level and retry the FOP.

This is the same optimistic resolution/retry logic Linux kernel
resolver is proposing to use.

Change-Id: Iecbc25248754caf0915cd7205910563ff88ce5e7
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4928
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>