Fixup compiler warnings (see description)

- iocp: removed unused variable
- timer-ng: place parenthesis around bitwise comparison
- mcreq: replace hdrsize parameters as being uint8_t rather than
  lcb_size_t. Header sizes should not be bigger than 256 (There is
  extlen which technically allows the header to be 24+256 bytes in length
  but we'll ignore that for now in the request headers and address this
  once an actual command comes along with that extras size)

- LCB_NS2US macro should cast result to uint32
- retryq.c: backoff factors should be cast to floats
- mctests: use unsigned for loop iterator when bound is unsigned
- t_slist: Compare int to int and bool to bool
- rdb/t_basic: compare unsigned to 0U (!)

Change-Id: I863f043ca8bca89ee02284be57d3e6dd309e5ae0
Reviewed-on: http://review.couchbase.org/36396
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>