feat: allow source and destination clusters to overlap

[levkk]

Allow source and destination clusters to overlap. Matching databases in the destination cluster are ignored and data is only routed to shards not specified in the source cluster. This little hack allows us to:

1. Sync omni tables to new shards added to an existing cluster
2. Copy rows from existing nodes to new nodes with new sharding map

Example

[[databases]]
name = "source"
host = "10.0.0.0"

[[databases]]
name = "destination"
host = "10.0.0.0" # Same host as source!
shard = 0

[[databases]]
name = "destination"
host = "10.0.0.1"
shard = 1

[[sharded_tables]]
database = "destination"
column = "tenant_id"

[[sharded_tables.mapping]]
values = [1, 2, 3]
shard = 0

[[sharded_tables.mapping]]
values = [4]
shard = 1

Using data-sync, we can copy:

1. all omni tables
2. all rows matching tenant_id = 4

from source to destination, without touching the original node.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...src/backend/replication/logical/subscriber/copy.rs	69.69%	10 Missing ⚠️
...c/backend/replication/logical/subscriber/stream.rs	93.47%	3 Missing ⚠️
.../replication/logical/subscriber/duplicate_check.rs	90.90%	1 Missing ⚠️

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