BUG-5280: remove support for talking to pre-Boron clients

Both transaction chains and transaction identifiers are being changed to
structured data. Unfortunately there is no way to reliably map string
identifiers from versions prior to Boron in a consistent manner.

Pre-Boron messages do not have a concept of a frontend identifier nor its
generation. Implementations are assigning them in a non-deterministic
order and rely on timestamps to disambiguate between generations.

Since Shard-internal state keeping and replication logic will require
these concepts, this patch marks a clean break in compatibility. This
compatibility is only needed in mixed-version clusters, which have not
been validated to work and even if the data store works, it cannot
cope with model mismatches expected from differing versions.

Change-Id: I168fa0a98be10aebd680ce3323c458aad7f15865
Signed-off-by: Robert Varga <rovarga@cisco.com>