Flatten distributeTimeseriesToReplicas data structures in receive/handler.go

[gavins-db]

In some previous profiling I learned that distributeTimeseriesToReplicas which returns a map[endpointReplica]map[string]trackedSeries, map[endpointReplica]map[string]trackedSeries is allocating more objects than necessary due to that pattern, similarly, allocating a replicas []uint64, slice rather than just iterating between two numbers is also unnecessarily wasted memory.
These are micro optimizations and are still largely overshadowed by other concerns but these changes are foundational to #306 (my vt migration) and really should just be pulled forward here. Furthermore, at least in my opinion, this makes what's happening under the hood significantly more easy to read and reason about.

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• Introduce distributionKey struct to replace nested map[endpointReplica]map[string]trackedSeries with flat map[distributionKey]*trackedSeries
• Use replicaStart/replicaEnd range instead of []uint64 replicas slice
• Remove remoteWriteParams; pass replica and timeseries directly
• Update gatherWriteStats, sendWrites, sendLocalWrite, sendRemoteWrite signatures to use the flattened map and pointer trackedSeries
• Update TestDistributeSeries for new signatures

Verification

• make test-local