zlogger: use cache for gettid() to reduce logging hot path overhead

zlogger: use cache for gettid() to reduce logging hot path overhead

Use thread local variable to cache gettid() calls and clear it with
pthread_atfork() hook. This reduces the logging hot path overhead by
avoiding any kernel calls.

On RPi4 this reduced a test message logging time (averaged) from 2.2us
to 0.8us.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I8ed74a4442d76183f2403861ac3c428fd9b5b051