Reduce unnecessary clears of legacy version space

[alecgrieser]

Whenever we perform check version, if the meta-data or format version have changed, we try and clear the old version subspace if the new meta-data doesn't store record versions:

fdb-record-layer/fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/provider/foundationdb/FDBRecordStore.java

Lines 3921 to 3928 in a5f98c4

	if (metaDataVersionChanged) {
	// Clear the version table if we are no longer storing record versions.
	if (!metaData.isStoreRecordVersions()) {
	final Transaction tr = ensureContextActive();
	tr.clear(getSubspace().subspace(Tuple.from(RECORD_VERSION_KEY)).range());
	}
	info.setMetaDataversion(newMetaDataVersion);
	}

This is done because if we change the meta-data from storing to not storing record versions, than we will never consult this subspace again, and so if we don't clear it out, we'll leave stale data around. Worse, this can actually could cause records to be associated with the wrong versions if:

1. A record is stored with a version
2. Version storage is turned off, but the version data not cleared
3. That record is updated (or the record is deleted and a new record is stored at the same primary key)
4. Version storage is turned back on
5. The record written in step 3 now will be associated with the version saved in step 1

Note that this is not a concern with the newer version format, where versions are stored next to the record, as in that case, the version is always read, even if the meta-data option to store records with versions is not enabled. (The option effectively controls whether or not versions are added by default, not whether they are stored and read at all, like it does in the old format version.)

We can reduce the number of range clears here by being more exact about when we issue this clear range. For example, we never need to clear this out on new stores (as the range is already empty), nor do we need to if the store was not using the old version subspace before the meta-data or format versions are updated. We also don't need to clear this out if the old meta-data did not store record versions, but that information is unfortunately not available during checkVersion, because it doesn't have a copy of the old meta-data (just the old meta-data version).