Refactor Server class

[robocoder]

Rationale:

• the Server class is getting bloated
• the Server class has too many public functions:
  • compileFile() - always recompiles
  • checkedCompile() - compiles scss if cache doesn't exist
  • checkCachedCompile() - recompiles if necessary
  • serve() - the original purpose of this class

Identified candidate cache implementations:

• never - no caching; always (re)compiles
• read - read only if cache exists; doesn't create
• once - compiles only if cache doesn't exist
• surface/shallow - recompile if main .scss file changes (note: this has no current API equivalent)
• deep - recompile if main .scss file or any of its imports changes

Technical considerations:

• bust up the Server class
• Cache/*.php implementations
• "deep" should detect changes in setVariables() and setImportPaths()

Other changes:

• make Compiler cache aware (also, see existing importCache)
• stale file detection using algorithm instead of metadata Update Server with cachedCompile() method like lessphp #321
• parse tree cache Is there a way to make the compiler more performant? #328
• register custom error handler Ability to register error handler with Server #330
• register custom input finder Manually specify file to be served #329
• option to set/override or remove comment "compiled by scssphp..." Option to remove comment "compiled by scssphp..." #477
• option to compress (gzip) output Function to get gzipped result #483