SourceLayout: refactor regex pattern objects

SourceLayout: refactor regex pattern objects

* moves the regex pattern state storage to class RegexPattern
  in base/RegexPattern.h which is MEMPROXY_CLASS pooled and
  constructed with flags and pattern preset.
 - for now the regcomp generated data is set separately.

* Replaces ACL storage class RegexList with a std::list

* converts refresh_pattern regex data to class RegexPattern for
  its pattern and -i/+i flag details.