Remove the default index

The default index, `_yz_default`, was added to prevent AAE from doing
unnecessary repairs after tree re-build.  After a Yokozuna hashtree is
cleared it is rebuilt from data in Solr.  If Solr only contains data
for indexed buckets then its hashtree will be missing hashes for
non-indexed buckets.  Since AAE exchanges based on partition the
system will think keys are missing and potentially invoke lots of
repair.  The default index avoids this by indexing the needed data for
AAE for buckets which are not indexed.  AAE works, but at the cost of
affecting write throughput on non-indexed buckets and increasing the
index size.  On my local workstation the default index reduced write
throughput to 56% of baseline.

This tradeoff is unnecessary.  Write throughput on a non-indexed
bucket can remain much closer to baseline without causing trouble for
AAE.

* Determine if the object needs to be indexed.

* If it does then write to Solr and update the hashtree.  If it
  doesn't then just update the hashtree.  This ensures that the
  hashtrees are kept up to date with significantly less cost than
  sending an update to Solr

* After a Yokozuna hashtree is cleared it will be missing keys stored
  in non-indexed buckets.

* During exchange each missing key will cause the repair function to
  be invoked.

* This repair function will now check if the object needs to be
  indexed.  I.e. an actual missing index entry versus an object that
  doesn't need to be indexed.  If the object is not to be indexed then
  the Yokozuna hashtree is simply sent an update.

* So the first exchange of a tree after re-build will invoke the
  repair function potentially many times.  But these invocations will
  consist of a simple check + a hashtree update.  This is far less
  work than minimally indexing all data.
Remove references to default index

The default index no longer exists.
Fix typo
Update meck because other repos expect 0.8.1
Fix repair count

Only count "full" repairs towards total count.  A `full_repair` is one
that actually repairs a missing or invalid Solr document.  A
`tree_repair` is one that only updates the Yokozuna hashtree because
the `BKey` is not stored in an indexed bucket.  In that case only the
tree should be updated to keep AAE happy.  A `failed_repair` is one
where the repair failed.  These can be ignored as the next exchange
will retry.
Extract AAE test into its own module

Move the AAE test out of essential and into its own module.  This
allows a more focused and faster running test and simplifies the
essential test as well.

Change `random_keys` to prevent returning 0 keys.
Verify repair count in AAE test

Verify that repair counts are correct after a full round of exchange.
Prevent indefinite repair of non-indexed objects

Even in the case of a non-indexed bucket the object must be read in
order to build the correct hash for the hashtree.

Add a test to verify indefinite repair is not occurring.

Add a utility function in `yz_rt` to count the number of calls to a
function.
correct variable name change in previous commit
comment typo: to -> can
Merge pull request #171 from basho/feature/rz/remove-default-index

Remove the default index