`File::lock()` and related functions are unsupported on some platforms

[s-cerevisiae]

Describe the bug
std file-locking APIs are not supported on some platforms like Android. Creating databases always returns kind: Unsupported IOError, making fjall 3.0.0 unusable on these platforms.

To Reproduce
Steps to reproduce the behavior:

1. Run Database::builder("foo").open().unwrap() on platforms like Android
2. It errors out with Io(Error { kind: Unsupported, message: "try_lock() not supported" })

Expected behavior
It should still be possible to create and load databases on these platforms despite file locking is currently not supported there.

Additional context
Maybe a valid behavior is to skip locking and log the event when locking is unsupported?

[marvin-j97]

Android support is an upstream issue: rust-lang/rust#148325

[s-cerevisiae]

I don't think std support of the function should be a blocker for platform support of this library. The detection of process contention is nice to have, but the database should work without being overly defensive when the lock can't be checked. Fjall 2.x works fine on Android, so I consider this a regression.

[Mivik]

IMO, we could add an allow_missing_flock flag so that Fjall can run on platforms without flock support (with a warning) instead of returning an error.

[marvin-j97]

Related: #258

[RalfJung]

FWIW, once Rust 1.98 gets released, std file locking should work on Android. :)
(If someone could test that with the latest nightly to ensure it really works, that'd be great.)