Add AAE status subsystem + finalize AAE for Riak 1.3 release

[jtuple]

This pull-request adds code to track the status of AAE as well as provides a console command for use with riak-admin to print out the status.

This pull-request also fixes a bug with AAE, where tree build times were not persisted to disk, and therefore restarting a node would delay tree expiration. Build times are now persisted.

Finally, this pull request removes some extremely spammy log messages that were already flagged with TODOs in the source. Commenting out these messages seems like the best approach for user-experience for Riak 1.3. We can re-consider for Riak 1.4.

The approach used by the status subsystem (recomputing everything on each request for status) is rather inefficient and can easily be optimized to both cache certain values as well as update other aggregate information in real-time as status events occur. However, this is largely premature optimization for a part of code that is not critical. The current solution is straightforward and easily handles things like changing bucket N-values, changing ring ownership, etc without any special code needed (such as to flush cached values in a cached approach). Simply put, let's ship a working solution now, consider optimizing for Riak 1.4.

[jtuple]

This code relies upon a fix to riak_core_format:human_time to provide correct times in the status output. That fix is done in the sibling pull-request: basho/riak_core#261

[jtuple]

Note: When testing this for review, you'll likely want to adjust the AAE options in app.config. The defaults of building 1 tree per hour will take quite while to see AAE exchanges really take off.

[jtuple]

Let me describe the status output.

The first output lists information about exchanges that occur for a given index.

================================== Exchanges ==================================
Index                                              Last (ago)    All (ago)   
-------------------------------------------------------------------------------
114179815416476790484662877555959610910619729920   10.1 min      8.0 hr       

The Last column shows when the last exchange for that index occurred. For example,
the last exchange occurred "10.1 min ago". The All shows how long it's been since
the partition has synced with all it's siblings. Ignoring multi-replica failures,
this essentially shows how recently all data on a given index was fully verified
to be correct and valid. This is likely the most important and interesting number
in all of AAE.

The second output section shows how long ago a given index's hash trees were built.
By default, Riak expires hash trees once a week to ensure the hash tree and on-disk
data stay in sync, as well as to detect bit rot and silent disk corruption.

================================ Entropy Trees ================================
Index                                              Built (ago)
-------------------------------------------------------------------------------
114179815416476790484662877555959610910619729920   12.3 hr

The section shows statistics about the number of keys repaired in any given
exchange, showing the number of repairs in the most recent exchange, as well
as the mean and max value ever seen since node restart.

================================ Keys Repaired ================================
Index                                                Last      Mean      Max   
-------------------------------------------------------------------------------
114179815416476790484662877555959610910619729920      0         8         10

[jrwest]

+1