Add engine.Key.Address() method for addressing local keys.

Local keys are keys which are located on the same range as other keys
minus a local key prefix. Local key prefixes take the form:
"\x00\x00\x00[4 characters]". Any commands addressed to keys of this
form are direct to the range containing the same key minus the prefix.
Transaction records, response cache entries, range metadata (and
in the future others) all have local keys.

Changed cases where we write directly to the underlying engine to use
binary-encoded keys. Now that we binary encode all keys, need to pass
correct prefixes to the C++ custom compaction filter for GC. In this
way, we avoid having C++ need implementations of our decoding methods.

Moved a number of methods into methods of struct engine.Key. Added
associated unittests.
Since we're now using random values, we are triggering snappy to disable
compression. Make this test more flexible and allow it to assume
compression will be disabled due to random value input.
Updates for Tobias' comments. Added key checking to the REST API.
backed out changes to rest.go--this permission check is done automatically in dist_kv.go and system keys will only be accessible to user root
Merge pull request #100 from cockroachdb/spencerkimball/proto-key

Add engine.Key.Address() method for addressing local keys.
Merge pull request #101 from cockroachdb/spencerkimball/approx-size

Since we're now using random values, we are triggering snappy to disable
Added simple offset measurement between two connected nodes
Updated rpc to use the server's hlc clock
Updated client_test in response to PR comments
Address rpc/client cleanup comments