Update SLP transaction history endpoints

[SpendBCH]

• Support fromBlock parameter to exclude already seen transactions
• Normalize result format
• Update swagger docs

[coveralls]

Pull Request Test Coverage Report for Build 2788

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 67 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 66.91%

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Files with Coverage Reduction	New Missed Lines	%
dist/routes/v2/services/slpdb.js	4	75.42%
dist/routes/v2/slp.js	63	46.37%
<!--	Total:	67		-->

Totals
Change from base Build 2784:	0.01%
Covered Lines:	2540
Relevant Lines:	3576

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💛 - Coveralls