Write lock never released

I was experimenting to understand the code base better and discovered that a device handle write lock is never released if [mut_func](https://github.com/cloudflare/boringtun/blob/master/boringtun/src/device/dev_lock.rs#L89) `panic!`s. As a result, the `DeviceHandle`'s remaining `event_loop` threads are blocked from doing any other work.

Generally it's not an issue unless someone is working on [api::api_set()](https://github.com/cloudflare/boringtun/blob/master/boringtun/src/device/api.rs). On MacOS, I see my application "freeze" and I have to `sudo kill` (not sure why but `ctrl-c` doesn't work once the condition is hit) the process which results in the `utun` interface hanging around until I reboot.

Start in boringtun/src/device/dev_lock.rs at mut_func and trace the write-lock handling when the callback panics; then inspect boringtun/src/device/api.rs around api::api_set(). Reproduce the panic path if possible and verify that the remaining event_loop threads can continue working and the device handle does not remain blocked.

Written by the indexing model from the issue text.