replication: introduce heartbeat request structs

The patch adds structs applier_heartbeat and relay_heartbeat.

It is similar to the previous commit, which does the same for all
the other replication requests, but these work not via struct
replication_request.

The heartbeats have their own codecs. The goal is to avoid the big
expensive replication_request usage with a lot of optional fields.
It makes sense, because the HBs are sent somewhat frequently.

They also store members by values. But copying before encoding and
decoding is avoided by storing the HB structs by values right
inside applier and relay.

The motivation is to simplify addition of a new field to
applier_heartbeat in a next commit.

Needed for #7253

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=refactoring