Migrate builtin I/O plugins to use 'v2'

This changes the plugins to use the 'v2' plugin interface, which employs
the `lcb_io_procs_fn` interface to call into the plugin to get the
further definitions for various socket APIs.

This is needed to support dead socket detection

Change-Id: I61470ea1521024c4d50f822674e614a4ebf78cc3
Reviewed-on: http://review.couchbase.org/43028
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>