D14N client to read from backend

[fbac]

Context

Regarding to sync performance and checking which groups have new messages without querying each group individually, the d14n backend includes APIs that can increase performance for those operations.

There will be a phase between fully migrating to read-write the d14n backend where the V3 client writes to V3 and reads from D14N to check which groups have new messages.

This also aligns with the testing strategy that no data is lost during migration.