select: stylistic changes

While trying to fork the select plugin, I realized the original could
do with some better naming conventions and use of proper typedefs for
handler types.

None of the logic was actually changed, just some whitespace and naming
fixes:

- We should use either unqualified or sel_-prefixed functions. There is
  no reason for each function to be prefixed with lcb_io_foo. Not only
  does this make the name longer, it also makes it sound like this
  function is actually exported somewhere rather than being static.

- Use prefixed names rather than the '_t' suffix.

Change-Id: I2b7f6a50a36ddaf7e30ef00425bcfaded175ccef
Reviewed-on: http://review.couchbase.org/39054
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>