Rename install_hook to set_index_flag

[coderoshi]

Now that all data is indexed (otherwise AAE won't work) there is no longer a notion of a hook. All data written to KV is passed to Yokozuna for minimal indexing. But now there is a flag yz_index_content to indicate to Yokozuna that the content should also be indexed.

• Rename the code in the appropriate places: yz_kv and yz_wm_index
• Update documentation, explain that everything is indexed and what the index flag does.

[rzezeski]

• Let's get rid of the two functions above and instead use set_index_flag/2 directly.
• Add a doc explaining that the index flag indicates whether or not the KV object's value should be indexed. If false only the minimal amount of information about the object is indexed so that Yokozuna knows about it and AAE can function correctly. If true then the object's value will be indexed as well.
• Add spec.

[rzezeski]

Sorry, I came off like a prick with that last comment. I'm just worried about tests like these given what I've seen in riak_kv over the last 2 years. Yokozuna is a new project. I'd like to try something different this time around. I'd like to keep all eunit and EQC tests in Yokozuna as pure as possible. I.e. only test pure functions or functions that require the most minimal of state setup that is to setup and teardown. I'd like to push all functional testing out to riak_test (this means the EQC test you wrote would have to move into riak_test, or somewhere other than the eunit tests).

I like keeping eunit/EQC pure because

• it avoids global state (ETS tables, gen_severs, etc) of one test from interfering with another
• the tests run faster
• the tests have less chance of randomly breaking thus people actually run them, a lot of people ignore the failing riak_kv tests because they are so unpredictable
• it's easier to write tests against (almost) pure functions

[rzezeski]

+1 to merge