iproto: fix deadlock when dropping connections from iproto

Deadlock is described in detail in the ticket. Let's don't wait while
connection that called iproto_drop_connections is deleted. It will be
deleted eventually when request executing iproto_drop_connections is
finished and all other requests in progress from this connection are
finished too (they are cancelled just as requests of other connections).

CE part of https://github.com/tarantool/tarantool-ee/issues/609

NO_DOC=bugfix
NO_CHANGELOG=feature is not released yet
NO_TEST=test is in EE