Move transaction combining from signrawtransaction to new RPC

Create a combinerawtransaction RPC which accepts a json array of hex raw
transactions to combine them into one transaction. Signrawtransaction is changed
to no longer combine transactions and only accept one transaction at a time.

src/rpc/client.cpp

@@ -95,6 +95,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "signrawtransaction", 1, "prevtxs" },
     { "signrawtransaction", 2, "privkeys" },
     { "sendrawtransaction", 1, "allowhighfees" },
+    { "combinerawtransaction", 0, "txs" },
     { "fundrawtransaction", 1, "options" },
     { "gettxout", 1, "n" },
     { "gettxout", 2, "include_mempool" },