Add zf_log to performance tests

On my machine I'm getting following results (real time, best of 3):
        1 thread   10 threads   100 threads
zf_log   0.466s     2.711s       2.608s
spdlog   0.747s     5.121s       5.376s

Added CLOCK_MONOTONIC trick

2. Improved file_helper::exists() performance under linux to use stat sys call
3. Added unit tests

Fixed missing #pragma once

Fixed shadow warning in gcc

spurious character broke compilation