HPCC-12561 Implement 1st set of xml read improvements

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
This sharedResult protected by crit seems like a bad idea in general (especially once you have the StringBuffer local fix in place).

There is similar code in readBool that might not really need to go via cnv2Latin either?