internal/manifest: genericize btree

Convert the B-Tree used for organizing and reference counting TableMetadata
into a generic collection. This is in preparation for storing blob file
metadata in a B-Tree for copy-on-write reference counting. The iterator type is
left specialized to *TableMetadata because we will not need arbitrary iteration
over the blob file B-Tree.

We can consider eventually using an alternative B-Tree implementation, like the
one in the btreemap package, but we would need to introduce support for
copy-on-write reference counting. The existing table metadata B-Tree
implementation supports the reference counting semantics we need.

Author: jbowens

internal/manifest/annotator.go

@@ -79,7 +79,7 @@ type annotation struct {
 	valid atomic.Bool
 }
 
-func (a *Annotator[T]) findExistingAnnotation(n *node) *annotation {
+func (a *Annotator[T]) findExistingAnnotation(n *node[*TableMetadata]) *annotation {
 	n.annotMu.RLock()
 	defer n.annotMu.RUnlock()
 	for i := range n.annot {
@@ -92,7 +92,7 @@ func (a *Annotator[T]) findExistingAnnotation(n *node) *annotation {
 
 // findAnnotation finds this Annotator's annotation on a node, creating
 // one if it doesn't already exist.
-func (a *Annotator[T]) findAnnotation(n *node) *annotation {
+func (a *Annotator[T]) findAnnotation(n *node[*TableMetadata]) *annotation {
 	if a := a.findExistingAnnotation(n); a != nil {
 		return a
 	}
@@ -111,7 +111,7 @@ func (a *Annotator[T]) findAnnotation(n *node) *annotation {
 // nodeAnnotation computes this annotator's annotation of this node across all
 // files in the node's subtree. The second return value indicates whether the
 // annotation is stable and thus cacheable.
-func (a *Annotator[T]) nodeAnnotation(n *node) (t *T, cacheOK bool) {
+func (a *Annotator[T]) nodeAnnotation(n *node[*TableMetadata]) (t *T, cacheOK bool) {
 	annot := a.findAnnotation(n)
 	// If the annotation is already marked as valid, we can return it without
 	// recomputing anything.
@@ -153,7 +153,7 @@ func (a *Annotator[T]) nodeAnnotation(n *node) (t *T, cacheOK bool) {
 // files in the node's subtree which overlap with the range defined by bounds.
 // The computed annotation is accumulated into a.scratch.
 func (a *Annotator[T]) accumulateRangeAnnotation(
-	n *node,
+	n *node[*TableMetadata],
 	cmp base.Compare,
 	bounds base.UserKeyBounds,
 	// fullyWithinLowerBound and fullyWithinUpperBound indicate whether this
@@ -237,7 +237,7 @@ func (a *Annotator[T]) accumulateRangeAnnotation(
 
 // InvalidateAnnotation removes any existing cached annotations from this
 // annotator from a node's subtree.
-func (a *Annotator[T]) invalidateNodeAnnotation(n *node) {
+func (a *Annotator[T]) invalidateNodeAnnotation(n *node[*TableMetadata]) {
 	annot := a.findAnnotation(n)
 	annot.valid.Store(false)
 	if !n.leaf {