ngtcp2's top-level CMakeLists.txt is not friendly to being used as a
subdirectory (for instance, it always adds a `check` target when cunit
is installed), so stop using it in favour of skipping directly into the
ngtcp2/lib subdir.
This requires some hackery to set up a couple of the things the
top-level cmake does, but it isn't *too* painful.
- Move uvw target to external instead of llarp/CMakeLists.txt
- Add sqlite_orm headers via interface library rather than shoving it
into the global include search path.
- Find sqlite3 as part of the sqlite_orm target deps rather than linking
it separately. (Also don't link it into lokinet-util, which doesn't
appear to actually need it).