Consolidate benchmark code to benchmarks/.

Currently, the benchmark used to assess leveldb changes lives in db/. The codebase also contains two benchmarks against other database engines in doc/bench/. Moving all the benchmarks in one place opens up the way for extracting common code.

PiperOrigin-RevId: 246737541

.clang-format

@@ -8,7 +8,7 @@ DerivePointerAlignment: false
 # Order them so that when imported to the authoritative repository they will be
 # in correct alphabetical order.
 IncludeCategories:
-  - Regex:           '^(<|"(db|helpers)/)'
+  - Regex:           '^(<|"(benchmarks|db|helpers)/)'
     Priority:        1
   - Regex:           '^"(leveldb)/'
     Priority:        2