libglusterfs/rot-buffs: rotational buffers

This patch introduces rotational buffers aiming at the classic
multiple producer and multiple consumer problem. A fixed set
of buffer list is allocated during initialization, where each
list consist of a list of buffers. Each buffer is an iovec
pointing to a memory region of fixed allocation size. Multiple
producers write data to these buffers. A buffer list starts with
a single buffer (iovec) and allocates more when required (although
this can be preallocatd in multiples of k).

rot-buffs allow multiple producers to write data parallely with
a bit of extra cost of taking locks. Therefore, it's much suited
for large writes. Multiple producers are allowed to write in the
buffer parallely by "reserving" write space for selected number
of bytes and returning pointer to the start of the reserved area.
The write size is selected by the producer before it starts the
write (which is often known). Therefore, the write itself need not
be serialized -- just the space reservation needs to be done safely.

The other part is when a consumer kicks in to consume what has
been produced. At this point, a buffer list switch is performed.
The "current" buffer list pointer is safely pointed to the next
available buffer list. New writes are now directed to the just
switched buffer list (the old buffer list is now considered out
of rotation). Note that the old buffer still may have producers
in progress (pending writes), so the consumer has to wait till
the writers are drained. Currently this is the slow path for
producers (write completion) and needs to be improved.

Currently, there is special handling for cases where the number
of consumers match (or exceed) the number of producers, which
could result in writer starvation. In this scenario, when a
consumers requests a buffer list for consumption, a check is
performed for writer starvation and consumption is denied
until at least another buffer list is ready of the producer
for writes, i.e., one (or more) consumer(s) completed, thereby
putting the buffer list back in rotation.

[
   NOTE:
   I've not performance tested this producer-consumer model
   yet. It's being used in changelog for event notification.
   The list of buffers (iovecs) are directly passed to RPC
   layer.
]

Change-Id: I88d235522b05ab82509aba861374a2312bff57f2
BUG: 1170075
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/9706
Tested-by: Vijay Bellur <vbellur@redhat.com>

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