Change the table config in the microbenchmarks Bench.Database.LSMTree

Use no disk caching, since that makes things more interesting.

Use a much smaller write buffer. Otherwise the number of elements was
80k and the write buffer was a quater of that at 20k, which corresponds
to just four runs. That doesn't allow for much run merging, which makes
things less interesting. By using 1k for the write buffer, we get 80
runs inserted (or more for the benchmarks that use multiple iterations).

Author: dcoutts

bench/micro/Bench/Database/LSMTree.hs

@@ -79,8 +79,9 @@ instance ResolveValue V3 where
 
 benchConfig :: TableConfig
 benchConfig = defaultTableConfig
-    { confWriteBufferAlloc  = AllocNumEntries 20000
+    { confWriteBufferAlloc  = AllocNumEntries 1000
     , confFencePointerIndex = CompactIndex
+    , confDiskCachePolicy   = DiskCacheNone
     }
 
 benchSalt :: Bloom.Salt