chainhash: JSON marshal hash as string

[ffranr]

This PR adds a MarshalJSON() method to chainhash.Hash. As a result, when serializing structures as JSON, a chain hash will be represented as its String() value.

This change simplifies the JSON serialization of block and block header.

[coveralls]

Pull Request Test Coverage Report for Build 3604570378

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 55.172%

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Files with Coverage Reduction	New Missed Lines	%
connmgr/connmanager.go	2	86.07%
<!--	Total:	2		-->

Totals
Change from base Build 3516278895:	0.02%
Covered Lines:	26568
Relevant Lines:	48155

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💛 - Coveralls