Auto merge of #1060 - str4d:1033-randomise-nonce, r=ebfull
Auto merge of #1060 - str4d:1033-randomise-nonce, r=ebfull

Randomise the nonce in the block header

The top and bottom 16 bits of the nonce are left clear for local use as thread
flags and counters. This does not leak any more local information about the
miner than is currently exposed.

The cleared bits should not be considered a consensus rule, as miners are free
to set all bits of the nonce however they wish.

Closes #1033

CTRoundTable/Encrypted.Cash,bitcoinsSG/zcash,CTRoundTable/Encrypted.Cash,bitcoinsSG/zcash,bitcoinsSG/zcash,CTRoundTable/Encrypted.Cash,CTRoundTable/Encrypted.Cash,bitcoinsSG/zcash,CTRoundTable/Encrypted.Cash,bitcoinsSG/zcash,CTRoundTable/Encrypted.Cash,bitcoinsSG/zcash