Use msgpack for storage instead of gob

[patrick-ogrady]

Fixes #49

Motivation

The gob package requires all types to be registered. When Rosetta API implementations populate metadata (on any struct) with non-primitive values, it caused gob to throw an error:

gob: type not registered for interface: *

Solution

Instead of registering all non-primitive types we could expect to see in Metadata (map[string]interface{}, map[int]interface{}, []interface{}, etc), the internal/storage package has been modified to store data as msgpack using this package.

Not only will this change resolve #49, it also improves performance (compared to gob). Alternatively, we could have replaced gob with json but this would've caused significant performance degradation.

NOTE: THIS CHANGE BREAKS ALL EXISTING STATE STORAGE! YOU WILL NEED TO CREATE A NEW DATASTORE WHEN UPDATING THE ROSETTA-CLI!

[coveralls]

Pull Request Test Coverage Report for Build 1420

• 18 of 18 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 75.385%

---

Totals
Change from base Build 1385:	0.5%
Covered Lines:	392
Relevant Lines:	520

---

💛 - Coveralls