appropriate

Initial plumbing work; needs tests and more client_channel
implementation.

The 'tr' command can actually accept escapes, such as \n.

Rename ThreadPool -> FixedSizeThreadPool to allow for later introduction
of DynamicSizeThreadPool

Use it to validate that arithmetic on time types makes even some vague
kind of sense.

Tests not yet included.

Still missing:
- Caching of the already checked JWTs (although it could be done at an
  upper layer).
- Caching of the jwks_uri to avoid 2 roundtrips for each verification.