[confbal] Rename and redesign a bunch of cuttlefish settings, improve docs.

* Setting name is removed from all docstrings. The name can be
  generated from the schema and will be displayed when using the
  "describe" cuttlefish command.
* sync_strategy -> sync.strategy; sync_interval -> sync.interval
* merge_window -> merge.policy -- There is no window if set to
  'always' or 'never', so "policy" seems a better name.
* merge_window.start/end -> merge.window.start/end
* frag_merge_trigger -> merge.triggers.fragmentation (more
  descriptive). The "is_percentage" validator was also added to
  constrain the values.
* dead_bytes_merge_trigger -> merge.triggers.dead_bytes (similar to
  frag_merge_trigger).
* frag_threshold -> thresholds.fragmentation (again adding
  "is_percentage" validator)
* dead_bytes_threshold -> thresholds.dead_bytes
* small_file_threshold -> thresholds.small_file
* max_fold_age -> fold.max_age. Added the possible value of
  `unlimited` (replaces -1 magic value) as well as a duration in
  milliseconds. The input value is upscaled to microseconds, but we
  have no evidence of anyone setting this setting. Probably best to
  change it in advanced.config.
* max_fold_puts -> fold.max_puts. Added value of `unlimited` which
  replaces the magic value of -1.
* Added `off` to expiry, which disables the feature (instead of the -1
  magic value).
* require_hint_crc -> hintfile_checksums. Changed settings to `strict`
  and `allow_missing` instead of true/false.
* expiry_grace_time -> expiry.grace_time
* multi_backend settings were converted to use @see tags instead of
  copying the docstrings.
Update schema tests for renamed settings.
Merge pull request #127 from basho/refactor/confbal/schema-renames

Rename/refactor a bunch of cuttlefish settings.