Avoid splitting coins when minting

[shane-kerr]

Over time as new coins are minted, the wallet accumulates a lot of small amounts which also mint, and eventually the wallet is constantly trying to mint very small amounts.

People manually collect these small amounts into a single transaction, which makes the minting go more smoothly.

However, it seems like there is no reason why we cannot just add these to the coin that minted them. This would prevent these small coins from accumulating.

[shane-kerr]

From https://github.com/MintcoinCommunity/MintCoin-Docs/blob/master/Announcements/2.0-Proposed_Revisions.md:

StakeSplitThreshold. Allow users to set a coin-value threshold for splitting an input upon minting. This will reduce the need to manually recombine resulting low-value inputs from the splitting that occurs when an input mints.