config: make I/O operations synchronous

[jordigh]

Original commit messages:

• a27eb98 config: replace fse functions with sync variants

  I need to be able to read the config at module load time, which makes
  async difficult if not impossible.

  This will make all config operations synchronous, which is fine. The
  file is tiny and seldom read or written to.

• 3268077 config: remove IReadableConfigValue type

  This type is only used to define the IWritableConfigValue type, and
  since we are now doing synchronous read and writes, there's no point
  in separating get from set.

• bfa045b config: remove all async/await around config functions

  Now that I/O is synchronous, there's no need to have any more
  async/await in regards to the config functions.

[Spoffy]

Generally looks fine on the read side, though I think it would be valuable to keep the write side asynchronous.

Read is done primarily during startup, so the impact of a slow file read is minimal.

Write however can be done at any point during operation, and a slow write could hang the entire server. So I think it's worth keeping write async

[jordigh]

Updated commit messages:

• 971b9c0 config: replace fse read functions with sync variants

  I need to be able to read the config at module load time, which makes
  async difficult if not impossible.

  This will make read config operations synchronous, which is fine. The
  file is tiny and seldom read.

• 5f23415 config: remove all async/await around config read functions

  Now that reading is synchronous, there's no need to have any more
  async/await in regards to the those config functions.