fix(bigtable/bttest): make table gc release memory

We had a problem where we were unable to run the emulator for extended periods in dev environments without giving it huge amounts of RAM, even if we had quite aggressive GC policies applied to the tables.
This fixes that by making GC delete empty rows and columns.

bigtable/bttest/inmem.go

@@ -1242,13 +1242,23 @@ func (t *table) gc() {
 		return
 	}
 
+	// It isn't clear whether it's safe to delete within the iterator, so we do
+	// not
+	var toDelete []btree.Item
 	t.rows.Ascend(func(i btree.Item) bool {
 		r := i.(*row)
 		r.mu.Lock()
 		r.gc(rules)
+		if len(r.families) == 0 {
+			toDelete = append(toDelete, i)
+		}
 		r.mu.Unlock()
 		return true
 	})
+
+	for _, i := range toDelete {
+		t.rows.Delete(i)
+	}
 }
 
 type byRowKey []*row
@@ -1334,7 +1344,15 @@ func (r *row) gc(rules map[string]*btapb.GcRule) {
 			continue
 		}
 		for col, cs := range fam.cells {
-			r.families[fam.name].cells[col] = applyGC(cs, rule)
+			cs = applyGC(cs, rule)
+			if len(cs) == 0 {
+				delete(fam.cells, col)
+			} else {
+				fam.cells[col] = cs
+			}
+		}
+		if len(fam.cells) == 0 {
+			delete(r.families, fam.name)
 		}
 	}
 }