Persists Term in the (bbolt) Backend

[ptabor]

Additional layer of protection, that allows to validate whether we start replaying log not only from the proper 'index', but also of the
right 'term'.

[gyuho]

lgtm.

Just making sure, these series of changes won't affect our compatibility right?

etcd 3.4 receiving snapshot from 3.5 with the new meta keys won't crash to parse snapshot, because those are just extra keys in bolt?

[ptabor]

lgtm.

Just making sure, these series of changes won't affect our compatibility right?

etcd 3.4 receiving snapshot from 3.5 with the new meta keys won't crash to parse snapshot, because those are just extra keys in bolt?

Yes. We just add a new key so 3.5 -> 3.4 is safe: both during operations and/or rollback.
I also don't depend on the new data in any way and are prepared for the keys to not be set, so the transition 3.4 -> 3.5 is safe.

This data will be very handy for v3.6.

[ptabor]

FTR: Main goal: #12913