Add new DB merging API to distributor BucketDatabase

Abstracts away how an ordered merge may be performed with the database
and an arbitrary sorted bucket sequence, with any number of buckets
skipped, updated or inserted as part of the merge.

Such an API is required to allow efficient bulk updates of a B-tree
backed database, as it is suboptimal to require constant tree mutations.

Other changes:
- Removed legacy mutable iteration API. Not needed with new merge API.
- Const-iteration of bucket database now uses an explicit const reference
  entry type to avoid needing to construct a temporary entry when we can
  instead just point directly into the backing ArrayStore.
- Micro-optimizations of node remover pass to avoid going via cluster
  state's node state std::map for each bucket replica entry. Now uses
  a precomputed bit vector. Also avoid BucketId bit reversing operations
  as much as possible by using raw bucket keys in more places.
- Changed wording and contents of log message that triggers when buckets
  are removed from the DB due to no remaining nodes containing replicas
  for the bucket. Now more obvious what the message actually means.
- Added several benchmark tests (disabled by default)

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