applier: add timeout to greeting read

A Tarantool server is supposed to send a greeting message right after
accepting a new client so the first thing an applier does after
connecting to the master is reads the greeting. It does this without
timeouts. The problem is that if by mistake we connect to a wrong
instance, which doesn't send anything to clients, the applier will hang
forever (until the remote closes the socket), without logging any
errors.

This may happen even with a valid Tarantool instance - if SSL encryption
is enabled on the master, but not on the client, because the SSL
protocol assumes that the client initiates a connection by writing to
the socket first (before the server).

Let's add a timeout to the operation reading the greeting. The timeout
is set to replication_disconnect_timeout(), after which a connection is
broken if the master doesn't send heartbeats for that long. Note, we
don't add a timeout to other read/write operations issued to initiate a
replication connection, because if we received a greeting and it's
valid, then the master is likely to be fine.

Closes #7204

NO_DOC=bug