valsep: introduce new generalized blob writer

generalizedValueSeparation is a new ValueSeparation implementation that combines
the existing implementations from writeNewBlobs and preserveBlobReferences. It
is able to function in either mode (writing new blobs or preserving all blob
refs).

Currently, we either write new blob refs or preserve blob refs when writing an
sst. In the future (for tiered storage), we may do both.

The next commits will replace the old implementations with the generalized
separator.

Author: xinhaoz

internal/base/internal.go

@@ -762,3 +762,10 @@ func (ikr *InternalKeyBounds) SetLargest(ik InternalKey) {
 	ikr.largestTrailer = ik.Trailer
 	ikr.userKeySeparatorIdx = len(smallestUserKey)
 }
+
+type StorageTier uint8
+
+const (
+	HotTier StorageTier = iota
+	NumStorageTiers
+)