rpc: adjust NEP5 transfers amount JSON marshalling
This committ fixes the difference between Go and C# nodes:
Go:
```
{
"jsonrpc" : "2.0",
"result" : {
"received" : [
{
"blockindex" : 65,
"txhash" : "0x394f851cf167d664c0dbcf98e2e64f2da23022fd7943dcb914492529de20a945",
"transfernotifyindex" : 0,
"timestamp" : 1605535020126,
"transferaddress" : "NUVPACMnKFhpuHjsRjhUvXz1XhqfGZYVtY",
"amount" : "29999999",
"assethash" : "0x668e0c1f9d7b70a99dd9e06eadd4c784d641afbc"
}
],
"address" : "NULwe3UAHckN2fzNdcVg31tDiaYtMDwANt",
"sent" : []
},
"id" : 1
}
```
C#:
```
{
"id" : 1,
"result" : {
"address" : "NULwe3UAHckN2fzNdcVg31tDiaYtMDwANt",
"sent" : [],
"received" : [
{
"transferaddress" : "NUVPACMnKFhpuHjsRjhUvXz1XhqfGZYVtY",
"timestamp" : 1605535020126,
"txhash" : "0x394f851cf167d664c0dbcf98e2e64f2da23022fd7943dcb914492529de20a945",
"blockindex" : 65,
"transfernotifyindex" : 0,
"amount" : "2999999900000000",
"assethash" : "0x668e0c1f9d7b70a99dd9e06eadd4c784d641afbc"
}
]
},
"jsonrpc" : "2.0"
}
```
8e29a200c0