MB-16056: Replace cb_assert in memcached.cc for arg checking
MB-16056: Replace cb_assert in memcached.cc for arg checking

Replace cb_assert() for argument checking with one of:

a) std::invalid_argument exception where the caller has failed to pass
   valid arguments.

b) Close the connection (and log an error) where the connection is no
   longer in a valid state.

c) Remove some asserts which are superflous - for example various
   executors are called from dispach_bin_command which already checks
   'c'.

Change-Id: Ide2ef15dafb9ad35839387cf607ee2a2580f23bc
Reviewed-on: http://review.couchbase.org/54981
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/memcached,couchbase/memcached,owendCB/memcached,daverigby/memcached,daverigby/memcached,couchbase/memcached,owendCB/memcached,daverigby/memcached,couchbase/memcached,couchbase/memcached,daverigby/kv_engine,owendCB/memcached,daverigby/kv_engine,owendCB/memcached