Micro optimization to couch_file

The following short patch moves the MD5 verification outside the couch_file server:

https://github.com/fdmanana/couchdb/commit/51c463d682c478dcb273bd88f1ef3046a709689f

Despite being apparently so insignificant (couch_util:md5/1 takes about 700us in my machine), I get this significant results with relaximation:

$ node tests/compare_write_and_read.js --wclients 100 --rclients 200 \
-name1 md5_out -name2 trunk \
-url1 http://localhost:5984/ -url2 http://localhost:5985/ \
--duration 120

run 1) http://graphs.mikeal.couchone.com/#/graph/5c859b3e7d1b9bd0488cfe271105130c

run 2) http://graphs.mikeal.couchone.com/#/graph/5c859b3e7d1b9bd0488cfe2711051bba

The documents used in the test have a size of about 1Kb.
If nobody has an objection, I'll commit this to trunk.