This doesn't really need to be fallible but forces propagation of a ton
of error handling - no shards is always a sign of something being very
wrong, and can be caught in the caller if it's for some reason an
acceptable state / can be recovered from.
The Sequencer (which will be renamed shortly) is a type that represents
a single sequencer/shard/kafka partition in the router.
In order to minimise confusion with all the various IDs floating around,
we have a KafkaPartition - this commit changes the Sequencer to return
the Kafka partition index as a typed value, rather than a usize to help
eliminate any inconsistencies.
As a side effect of these conversion changes, I've tightened up the
casting to ensure we assert on any overflows - we juggle a lot of
numeric types!
Dom Dwyer
Carol (Nichols || Goulding)