Fix "expectedtime" estimate returned by RPC call "getstakinginfo".

It was wrong before, using nTargetSpacing rather than nTargetStakeSpacing, and ignoring current difficulty.
Merge pull request #47 from l0rdicon/staging

Cleaning up importwallet code and rename functions. walletimport -> wall...
Merge pull request #48 from dooglus/better-staking-estimate

Fix "expectedtime" estimate returned by RPC call "getstakinginfo".
Thread safety header files