[WIP] Add async rpc call z_sendmany

[bitcartel]

For issue #1200
Depends on #1205 #1208 #1233

[bitcartel]

Notes:

Async queue class can have more than one worker (default) and this could be added as a conf option.
Everything is in memory only, nothing is persisted to disk.
z_sendmany currently checks the input parameters but does not send.

Example output:

TERMINAL% ./zcashd -daemon
Zcash server starting
TERMINAL% ./zcash-cli z_sendmany mt8nv6J71e9ck57ADVTwfSBmUz5XbZzttg '[{"address":"mt8nv6J71e9ck57ADVTwfSBmUz5XbZzttg", "amount":1.0}, {"address":"tnNKjG8vQDptwJ8Wvtpekiu6Kncyv47JVMHiKpnW9JTTQxGVe7C7z9pkSkEB6ptZmtPWSc9PcysLn4Geisk7prggdJcifUr","amount":3.451233141234234}]'
5eebb77c-e385-4277-aae3-d31a0dabb7c8
TERMINAL% 
z_sendmany: **************** DUMMY RUN *****************
z_sendmany: source of funds: mt8nv6J71e9ck57ADVTwfSBmUz5XbZzttg
z_sendmany: minconf: 1
z_sendmany: send 100000000 to mt8nv6J71e9ck57ADVTwfSBmUz5XbZzttg
z_sendmany: send 345123314 to tnNKjG8vQDptwJ8Wvtpekiu6Kncyv47JVMHiKpnW9JTTQxGVe7C7z9pkSkEB6ptZmtPWSc9PcysLn4Geisk7prggdJcifUr
z_sendmany: checking balances and selecting coins and notes...
z_sendmany: performing a joinsplit...
z_sendmany: attempting to broadcast to network...
z_sendmany: operation complete!
z_sendmany: ********************************************

TERMINAL% ./zcash-cli z_getoperationstatus 5eebb77c-e385-4277-aae3-d31a0dabb7c8
{
    "id" : "5eebb77c-e385-4277-aae3-d31a0dabb7c8",
    "status" : "success",
    "creation_time" : 1471461975,
    "result" : {
        "dummy_txid" : "4a1298544a1298544a1298544a1298544a129854",
        "dummy_fee" : 0.00010000
    },
    "execution_secs" : 7.00164264
}
TERMINAL% ./zcash-cli z_getoperationstatus 5eebb77c-e385-4277-aae3-d31a0dabb7c8
error: {"code":-8,"message":"No operation exists for that id."}
TERMINAL% ./zcash-cli stop                                                
Bitcoin server stopping

[bitcartel]

Closing due to git voodoo. PR moves to #1274