fop-lookup: check xattr before getting GF_CONTENT_KEY

dict_get() was always called to get GF_CONTENT_KEY, event if
xattr was NULL. This generates a lot of log messages.
Avoid crashes on failed writev and truncate fops

When a fop fails, it does not have the local->cbk set. Since writev
and truncate fops use fields from it, they crashed.
Execute all modifying fops on all subvolumes

Some modification fops can be answered as soon as the minimum number
of fragments has been received, however this has been temporarily
increased to IDA_REQUIRE_MAX to help track some other problems.
Removed ia_blocks check in ida_iatt_combine()

This check was not really needed and, in fact, it can be different
on different underlying file systems on files that are perfectly
healthy.
readlink: size sent to subvolumes was not initialized
Changes needed to compile against gluster 3.4
Added autotools
README.md: updated
Added license file