Split collection.rs into sub-modules

[ffuugoo]

Refactor collection.rs into:

collection/
└── mod.rs
└── collection_ops.rs
└── point_ops.rs
└── search.rs
└── shard_transfer.rs
└── snapshots.rs

collection/mod.rs is a bit less than 600 LOC, everything else is around 300 lines.

This PR is structured the same way as #2761 (see its description for further details).

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