iproto: rename disconnect cmsg to destroy

Disconnect cmsg is a message that is used by iproto
thread to notify tx thread that a connection is dead
and has no outstanding requests. That is its
tx-related resourses are freed and the connection is
deleted.

But the text above is clear definition of destroy. The
patch harmonizes cmsg name and its puprose.

Secondly, the patch is motivated by #3859 which
requires separate disconnect and destroy phases.

Needed for #3859