Revision 1043524

Calculate and verify MD5 digests outside of a couch_file server

This has a significant positive impact on the performance, both for readers and writers,
when there are several requests in parallel acessing the same database or view index file.

$ 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

Closes COUCHDB-980