Add -maxstakevalue=X config option to prevent outputs worth more than X

Add -maxstakevalue=X config option to prevent outputs worth more than X CLAM from staking.

I sometimes receive large outputs and forget to split them into smaller amounts before they stake once. Then I have to wait 8 hours until I can split them.

This lets me put:

    maxstakevalue=20

in clam.conf, and know that any outputs over 20 CLAM won't be considered for staking.