Local and remote throttling is now applied to merges, mutations, and backups.

A few interrelated changes are combined in this PR:

1. Added interface IThrottler that hides throttling details.
2. Added a possibility to combine multiple independent throttlers in one (see ThrottlerArray and addThrottler())
3. As we could use multiple independent throttlers, the way we count metrics has changed. Instead of incrementing ProfileEvents in places where a throttler is used, we attach ProfileEvents directly to the root throttler. If multiple throttlers are used, all related ProfileEvents will be updated.
4. Merges, mutations, and backups now use two independent throttlers, instead of one (see changelog entry). For example, merges that read from S3 are now accounted not only by max_merges_bandwidth_for_server throttler, but also by max_remote_read_network_bandwidth_for_server throttler.

Note that merges and mutations were accounted in remote/local throttler a long time ago, but after #57877 they obtained an independent throttler and were excluded from remote/local throttling (even if merge-specific or mutation-specific throttling was disabled). The same logic was applied for BACKUPs even earlier when backup throttling was introduced.

Changelog category (leave one):

• Backward Incompatible Change

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Previously, BACKUP queries, merges and mutations were not using server-wide throttlers for local (max_local_read_bandwidth_for_server and max_local_write_bandwidth_for_server) and remote (max_remote_read_network_bandwidth_for_server and max_remote_write_network_bandwidth_for_server) traffic, instead they were only throttled by dedicated server settings (max_backup_bandwidth_for_server, max_mutations_bandwidth_for_server and max_merges_bandwidth_for_server). Now, they use both types of throttlers simultaneously.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)