This introduces a better set of defaults for large databases. I've
experimentally determined that it results in much better throughput in a
couple of scenarios with large databases, but I can't give any
guarantees the values are always optimal. They're probably no worse than
the defaults though.
* add skeleton for lib/syncthing
* copy syncthingMain to lib/syncthing (verbatim)
* Remove code to deduplicate copies of syncthingMain
* fix simple build errors
* move stuff from main to syncthing with minimal mod
* merge runtime options
* actually use syncthing.App
* pass io.writer to lib/syncthing for auditing
* get rid of env stuff in lib/syncthing
* add .Error() and comments
* review: Remove fs interactions from lib
* and go 1.13 happened
* utility functions
