Simplify the MockCommand interface

Previously we had both an handleJson method and an execute
method. This patch removes the handleJson method so that
all new handlers only need to think about implementing
the execute methos (which they must implement)

Change-Id: I314418d74ad4abba217a86b79197b9945d6c8373
Reviewed-on: http://review.couchbase.org/27810
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>