cln-plugin: replace log with tracing

[tracing] is nearly a [drop-in replacement] for [log].
[tracing spans] are very useful in RPCs, they add context to logs.
tracing already supports [capturing log's events]. So the library users don't need to change their code immediately.

While tracing support [emitting log's events]. But it does not support spans, which are essential for tracing's experience.

[tracing]: https://docs.rs/tracing
[log]: https://docs.rs/log
[drop-in replacement]: https://docs.rs/tracing/latest/tracing/index.html#for-log-users
[tracing spans]: https://docs.rs/tracing/latest/tracing/index.html#spans
[capturing log's events]: https://docs.rs/tracing/latest/tracing/index.html#consuming-log-records
[emitting log's events]: https://docs.rs/tracing/latest/tracing/index.html#emitting-log-records