Add Relation#flush.
The creator tool passes the namespace argument down to coprocessors.
Rename coprocessor arg app -> askari, namespace -> askari.namespace.

We'll reserve anything under askari, and keep the rest clean.
Add `VersionSchema#relation_for_table_and_family`.

It locates a `RelationSchema` within a version based on table and
column family names.

This represents a cheap way for coprocessors to identify relations:
they use their owning table name and the family from any `KeyValue`
they find.
Here's RegionObserver::DSL, ::Application and ::Runner.

The `DSL` offers a sinatra-esque way to build up a set of coprocessor
callbacks, which accumulate in a hidden, associated `Application`
instance.  This `Application` will then be handed to a `Runner`, which
interfaces with our java `RegionObserver` implementation to drive
event dispatch.

Each kind of event (e.g. `:post_put`) registers a dispatcher
responsible for invoking handlers and folding results.

We choose to imitate HBase's own dispatch rules (across coprocessors),
so that multiple (say) post_put hooks function identically whether
they share the same askari coprocessor instance or not.  These can be
inspected in HBase source in `RegionCoprocessorHost`.

This dispatch process is nearly identical for all events, but three
key facts vary between them.

First, some events are expected to return a value, others do not.

Secondly the results of some handlers are expected to be fed into the
arguments of successive handlers, others not.  The identity (index) of
this "pivot" argument of course varies between events.

Finally, some events choose to respect the `#complete` method on their
contexts by terminating dispatch before later handlers get invoked.
Other events ignore `#complete.`

Also drop the lock/unlock coprocessor events, they're deprecated.