Encode `CrsqlSeq`, `CrsqlDbVersion` and `Version` as varints

[jeromegn]

Doing so both in transit and in memory should bring in significant benefits. For example: using a u64 for CrsqlSeq when they should most-often fit in a u8 is a waste.

[jeromegn]

With #189, this is only going to be relevant for bandwidth concerns.