Remove the implicit conversion from int64 to XRPAmount. The motivation for this
was noticing that many calls to `to_string` with an integer parameter type were
calling the wrong `to_string` function. Since the calls were not prefixed with
`std::`, and there is no ADL to call `std::to_string`, this was converting the
int to an `XRPAmount` and calling `to_string(XRPAmount)`.
Since `to_string(XRPAmount)` did the same thing as `to_string(int)` this error
went undetected.
* The `tx` command now supports min_ledger and max_ledger fields.
* If the requested transaction isn't found and these fields are
provided, the error response indicates whether or not every
ledger in the the provided range was searched.
This fixes#2924