Method Snapshot

Snapshots the last commit and returns it. Once a commit is 'snapshotted,' it is protected from deletion (as long as this IndexDeletionPolicy is used). The snapshot can be removed by calling Release(IndexCommit) followed by a call to DeleteUnusedFiles().

NOTE: while the snapshot is held, the files it references will not be deleted, which will consume additional disk space in your index. If you take a snapshot at a particularly bad time (say just before you call ForceMerge(Int32)) then in the worst case this could consume an extra 1X of your total index size, until you release the snapshot.