cache - allow access to old (outdated) entry when creating cache entry

this allows to reuse the SourceText object which will enable incremental
parsing/updating of syntax trees
buffersync - introduce a SourceTextService to allow buffer updates

Makes the Rolsyn compiler ask for source text from a service when building
syntax trees. Allows to record source text changes when a corresponding
messages is being received
buffersync - incorporate feedback
buffersync - incorporate feedback
buffersync - incorporate feedback
buffersync - sorted usings
Merge pull request #767 from jrieken/buffersync

dth - enable buffer updates outside the file save cycle