Add db.hello() shell helper

We cannot alias db.isMaster() to db.hello() because db.hello() should send a “hello” command to the server. This means the client will receive different command responses than they received with db.isMaster().

Replace all occurrences of the old helper with the new helper except in jstests/multiversion. These passthrough tests can run against a 3.2 and 3.4 binary.