fixed memory leak on async_service destruction.
fixed memory leak on async_service destruction.

BWallet/libbitcoin,libmetrocoin/libmetrocoin-consensus,libmetrocoin/libmetrocoin-consensus,GroestlCoin/libgroestlcoin,BWallet/libbitcoin,libtetcoin/libtetcoin-consensus,libtetcoin/libtetcoin-consensus,GroestlCoin/libgroestlcoin,swansontec/libbitcoin,swansontec/libbitcoin,kaostao/libbitcoin,BWallet/libbitcoin,libtetcoin/libtetcoin-consensus,libtetcoin/libtetcoin-consensus,swansontec/libbitcoin,Airbitz/libbitcoin,libmetrocoin/libmetrocoin-consensus,Airbitz/libbitcoin,kaostao/libbitcoin,Airbitz/libbitcoin,libmetrocoin/libmetrocoin-consensus,GroestlCoin/libgroestlcoin,Airbitz/libbitcoin,libmetrocoin/libmetrocoin-consensus,kaostao/libbitcoin,libtetcoin/libtetcoin-consensus,BWallet/libbitcoin,swansontec/libbitcoin,GroestlCoin/libgroestlcoin,kaostao/libbitcoin