write-behind: implement causal ordering and other cleanup

Rules of causal ordering implemented:

- If request A arrives after the acknowledgement (to the app,
  i.e, STACK_UNWIND) of another request B, then request B is
  said to have 'caused' request A.

- (corollary) Two requests, which at any point of time, are
  unacknowledged simultaneously in the system can never 'cause'
  each other (wb_inode->gen is based on this)

- If request A is caused by request B, AND request A's region
  has an overlap with request B's region, then then the fulfillment
  of request A is guaranteed to happen after the fulfillment of B.

- FD of origin is not considered for the determination of causal
  ordering.

- Append operation's region is considered the whole file.

Other cleanup:

- wb_file_t not required any more.

- wb_local_t not required any more.

- O_RDONLY fd's operations now go through the queue to make sure
  writes in the requested region get fulfilled before getting
  processed.

- O_SYNC fd's operations now go through the queue to make sure
  previously acknowledged writes on the file (via other fds) are
  fulfilled before getting processed.

- Option to not honor O_SYNC is now removed.

- Option to ignore O_DIRECT is added (useful when running a VM and the
  drive appears with NCQ/TCQ or WCE=1 for the guest.)

- Option to disable_first_nbytes is removed (as the cause of the
  bug which required this was diagnosed to be missing TCP_NODELAY.)

- General cleanup and better conformance to coding style and convention.

Change-Id: Ib44fb72da3727246b4a85174cb568c2f0231f6de
BUG: 857673
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: https://code.engineering.redhat.com/gerrit/1866
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>

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