Allow 24bit thread id in VanillaChronicle

Thread id was previously restricted to 16 bits which
caused failures in some environments.
Under linux the max pid can be set using the setting
/proc/sys/kernel/pid_max
Merge pull request #64 from brasslock/thread-id-24bits

Allow 24bit thread id in VanillaChronicle
CHRON-21 Support systems with a higher range of thread ids.