Cleaned up conversion of `int` and `long` in converters

[chrjohn]

trafficstars

Put parseLong and parseInt methods into IntConverter and used integer parsing method used in #529 that was also already present here for long: https://github.com/quickfix-j/quickfixj/blob/0cb4aa88f757933a5da52c98a6b9a0b1235e2a80/quickfixj-core/src/main/java/quickfix/field/converter/AbstractDateTimeConverter.java#L68-L74

NB: the old code used Integer.parseInt() which also allowed a plus sign as first character. This is not per the FIX spec:

Sequence of character digits without commas or decimals and optional sign character (characters “-” and “0” – “9” ).