engine: send vclock with 0th component during join

This commit makes engine to send vclock without ignoring 0th component
during join, which is needed for checkpoint FETCH SNAPSHOT.

Currently engine join functions are invoked only from
relay_initial_join, which is done during JOIN or FETCH SNAPSHOT.
They respond with vclock of the read view we're going to send.

In the following commit checkpoint FETCH SNAPSHOT will be introduced,
which responds with vclock of the checkpoint, we're going to send.
Such vclock may include 0th component and it's crucial to send it to
a client, as in case of connection failure, client will send us the
same vclock and we'll have to use its signature to figure out, which
checkpoint client wants.

So, we have to send and receive 0th component of the vclock during
FETCH_SNAPSHOT. This commit also introduces decoding vclocks without
ignoring 0th component, as they'll be used in the following commit too.

Needed for tarantool/tarantool-ee#741

NO_DOC=internal
NO_TEST=ee
NO_CHANGELOG=internal