Migrate to cell_once instead of lazy_static

[gakonst]

@bstrie wrote:

For about a year now the Rust constant-evaluation pipeline has been robust enough that lazy_static is entirely replaceable by the once_cell crate, which provides a macro-free way of doing what lazy_static once did.
(furthermore, the once_cell stuff is being considered for inclusion in std: rust-lang/rfcs#2788)

This should be easy and fairly straightforward.

The PR should also disable the filter for lazy static in our cargo audit call: https://github.com/interledger-rs/interledger-rs/blob/master/.circleci/config.yml#L50