fix on_getblocks

like this it was ignored by bitcoin core and others because it was
returning 501 blocks, it seems that we don't need to add head hash, and
just providing 500 children hashes is ok, it successfully sinced
bitcoin-qt client in this form
check for double spends within currently validated block
rename spent validation

since it returns true when funds are *not* yet spent
add test double-spending output from same block
Merge branch 'check_spent_in_current_block'
revert txout api change
Merge remote-tracking branch 'comboy/fix_getblocks'
Merge remote-tracking branch 'comboy/txout_script_revert'