Add rbuf_existing statistic

Add `rbuf_existing` statistic - number of times a read buffer was
already attached to a connection and hence didn't need a new buffer
allocating.

Change-Id: Ic93b941b33d61787b9bbaf1d3ec30183de781785
Reviewed-on: http://review.couchbase.org/39594
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Additional assertions that connection buffers have been freed

Change-Id: I13a88fc3f72e26251e31b7df9bc4689bab38f385
Reviewed-on: http://review.couchbase.org/39595
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Fix build break on win32 due to missing getopt

Change-Id: Ic54c1950398a822117b537db2f6f57f388e689b6
Reviewed-on: http://review.couchbase.org/39731
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Merge remote-tracking branch 'membase/3.0'

* membase/3.0:
  Fix build break on win32 due to missing getopt

Change-Id: I900866bfff5edec630cebb12f3d5bb5c75217fac
Merge remote-tracking branch 'membase/3.0.1'

* membase/3.0.1:
  Fix build break on win32 due to missing getopt
  Additional assertions that connection buffers have been freed
  Add rbuf_existing statistic

Change-Id: I04a5080c98760773ea8544527415e7f3ccacae08