Allow BDK to be built with env_logger version 0.10

[evanlinjin]

Describe the enhancement

As noted by @monaka in #1012, env_logger versions below 0.10 uses atty (which is unmaintained). BDK should allow projects with higher MSRVs to use env_logger.

@apoelstra suggested in #1012 (comment):

If the dep can really be bumped with no changes to the codebase, it may be possible to provide a ranged version that covers multiple major versions.

If this is even possible, we need to update CI to test for every version of env_logger in our specified range using cargo update --precise.

[LLFourn]

If this is even possible, we need to update CI to test for every version of env_logger in our specified range using cargo update --precise.

what? 🤣 🤣 Do we rely on the precise details of env_logger to communicate transactions surreptitiously between threads or something? How could env_logger even influence the outcome of a CI?

[LLFourn]

We can also just delete env_logger as a dev dependency. It's only used in examples in that we will delete before 1.0 (I hope anyway).