A cache for strings of frequently used integers

Avoided some temporary IntField creation to compute length.
In fact, the "isStringEquivalent" branch that I added is useless with previous commit on Message class, which directly compute length & checksum if possible.
Change some 'for each' list iteration by old loop over index to avoid iterator allocations, as the jvm will not always do it for us.

…rlesbr1/quickfixj into charlesbr1-fieldmap-length-and-checksum

- changed NumbersCache to only hold numbers up to 99999 since it should be sufficient for normal use cases
- removed NumbersCache.get(double) method since its usage is questionable as it will truncate a double to a long value