Redesign S2E's logging system

[vitalych]

Several problems with the current one:

• Too verbose getWarningsStream() << "..." could be shortened to WARN("msg" << "bla");
• Too slow. The stream is evaluated regardless of log settings.

Could have something like:

#define WARN(...)  if (level >= WARN_LEVEL) { getWarningsStream() << __VA_ARGS__; }

[vitalych]

Also, logging to console seems to slow down multi-core mode by a lot (e.g., see maze demo).