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'
Merge remote-tracking branch 'comboy/fix_getblocks'
Merge remote-tracking branch 'comboy/txout_script_revert'
fix typo in wallet send
only relay blocks once the storage is in sync
wallet: fix config option parsing
make sure script data elements are strings

fixes error with testnet tx 1ff8d1e58922816f28e62b2db2666c933e6f31c83c8a44fe066d2ecd1aa08bf6 which has a script of "OP_RETURN OP_CHECKSIG"