Rename socket argument in functions of plugin v1 plugin

Change-Id: Ie7ea38112ea9b0dd0d3e9d71e89206589fce26be
Reviewed-on: http://review.couchbase.org/29039
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>