Design discussion: avoid global state?

[andreeaflorescu]

A pain point with global state is that we cannot run unit tests in parallel (aka we run unit tests with RUST_TEST_THREADS=1) which also adds problems like we are out of memory because the state is not clean (also check #1138).

We should discuss if we should refactor Firecracker such that we don't have global state (METRICS, LOGGER, others?). This would also help in creating mock objects in tests.

[dianpopa]

We could start by removing lazy_static! usages all across Firecracker (right now we have it in mmds, logger and net_util). For the net_util crate, I think #1238 is a good place for removing the lazily evaluated statics.

[alindima]

The components with global state left are logger, metrics and version_map MMDS was tackled in this PR: https://github.com/firecracker-microvm/firecracker/pull/2920