[lib] allow dynamic strings in the cached feature

[loganek]

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #49 into master will increase coverage by 0.93%. The diff coverage is 93.22%.

@@            Coverage Diff            @@
##           master     #49      +/-   ##
=========================================
+ Coverage   68.56%   69.5%   +0.93%     
=========================================
  Files          60      60              
  Lines        1740    1751      +11     
=========================================
+ Hits         1193    1217      +24     
+ Misses        547     534      -13

Flag	Coverage Δ
#unittests	69.5% <93.22%> (+0.93%)	:arrow_up:

Impacted Files	Coverage Δ
lib/global_timeline.cpp	100% <100%> (+14.28%)	:arrow_up:
lib/feature_cached_string.cpp	93.1% <93.1%> (ø)
...r/include/hawktracer/parser/endianness_convert.hpp	77.27% <0%> (-2.04%)	:arrow_down:
lib/mutex.cpp	92.85% <0%> (-0.48%)	:arrow_down:
client/tracepoint_map.cpp	0% <0%> (ø)	:arrow_up:
client/converter.cpp	0% <0%> (ø)	:arrow_up:
lib/listeners/tcp_listener.c	0% <0%> (ø)	:arrow_up:
parser/event.cpp	83.92% <0%> (ø)	:arrow_up:
lib/platform/linux/cpu_usage.c	100% <0%> (ø)	:arrow_up:
client/chrome_trace_converter.cpp	0% <0%> (ø)	:arrow_up:
... and 19 more