benchmark: Add --databases to benchmark sqlite databases

Rescued from commit 11d6e2e260 which removed
db benchmarks in favor of benchmarking arbitrary git-annex commands. Which
is nice and general, but microbenchmarks are useful too.

Types/Benchmark.hs

@@ -5,11 +5,22 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
+{-# LANGUAGE CPP #-}
+
 module Types.Benchmark where
 
 import Annex
 import Types.Command
+#ifdef WITH_BENCHMARK
+import Criterion.Main.Options (Mode)
+#endif
 
 type BenchmarkGenerator = [String] -> Annex (IO ())
 
 type MkBenchmarkGenerator = [Command] -> BenchmarkGenerator
+
+#ifdef WITH_BENCHMARK
+type CriterionMode = Mode
+#else
+type CriterionMode = ()
+#endif