Make CheckIndex doChecksumsOnly / -fast as default

This issue is a spin-off from discussion in https://github.com/apache/lucene/pull/128

Currently CheckIndex defaults to checking both checksum as well as content inside each segment files for correctness, and requires -fast flag to be explicitly passed in to do checksum only. However, this default setting was there due to lack of checksum feature historically, and is slow for most end-users nowadays as they probably only care about their indices being intact (from random bit flipping for example).

This issue is to change the default settings for CheckIndex so that they are more appropriate for end-users. One proposal from @rmuir is the following:

1. Make -fast the new default.
2. The previous -slow could be moved to -slower 
3. The current behavior (checksum + segment file content - slow check) could be activated by -slow.