Index snapshots by capture pointer instead of capture ID in TrackMatrix

Currently, a snapshot in TrackMatrix is:

typedef map<size_t, FeatureBasePtr > Snapshot; // key: track Id.
map<size_t, Snapshot > snapshots_; // key: Capture Id.

This proposal is to replace the key from Capture Id to CapturePtr:

typedef map<size_t, FeatureBasePtr > Snapshot; // key: track Id.
map<CaptureBasePtr, Snapshot > snapshots_; // key: Capture pointer.