blockchain: set the lastflushtime when setting the lastflushhash

[kcalvinalvin]

On startup when the headers-first mode is off, when receiving the first block, the periodic flush will trigger. The lastflushtime wasn't set which resulted in the flush being triggered on the first block on restart.

Addresses #2087

[coveralls]

Pull Request Test Coverage Report for Build 7393874669

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 56.789%

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Totals
Change from base Build 7367368711:	0.005%
Covered Lines:	28979
Relevant Lines:	51029

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💛 - Coveralls