This enables backup workers to know the end version of the epoch. Additionally,
the master recovery only needs to deal with crashed backup workers by
recruiting new workers to backup the unfinished version range.
The spilling type is now pulled out of the request, and then stored on
LogData for later access, and persisted in the tlog metadata per tlog
generation.
It turns out that serializing types as Unversioned is a bit wonky.
This fixes#1214
The basic idea is that ProtocolVersion is now its own type. This
alone is an improvement as it makes many things more typesafe. For
each version, we can now add breaking features (for example Fearless).
After that, there's no need to test against actual (confusing) version
numbers. Instead a developer can simply test
`protocolVersion->hasFearless()` and this will return true iff the
protocolVersion is newer than the newest version that didn't support
fearless.
Jingyu Zhou