UNDERTOW-68 HTTP continue autmatic handler

This handler will automatically send a continue response
if an attempt is made to read from the channel.
Add toString() method to HttpServerExchange
Add ability to set options to the builder API
Update to latest servlet API
Log IOException at debug, as most of the time they are not interesting, and can provide a means to DOS the server
UNDERTOW-76 NPE in AsyncContexImpl.dispatch()
Bump timeout if setRequestTimeout is called
Return an empty collection rather than null
Fix up multipart size issue
Fix issue with canonical paths in async dispatch
Set empty roles semantic properly
UNDERTOW-77 Merge servlet roles together correctly

According to '13.8.1 Combining Constraints' we need
to take the union of the roles, not the intersection.