Use the Rust tracing crate for C++ logging
This PR swaps in the `tracing` crate (via FFI) for logging to either standard
output or `debug.log`. It transparently maps all existing `LogPrintf` and
`LogPrint` invocations to info-level `tracing` events, and passes through
correct file and line information. `error` invocations are mapped to error-level
`tracing` events, currently without line information (due to the way that
`error` is used in the codebase; swapping individual callsites to the new
`LogError` macro will provide that information).
The end-goal for this change is that we don't need to make any disruptive
changes to the codebase, but we can start to leverage `tracing`-specific
functionality where we want to, such as providing extra fields on certain log
lines (that can be filtered for), adding spans to record the flow of execution
through `zcashd`, and logging within C++ and Rust simultaneously. Support
for extra fields on spans and events will be added in a subsequent PR.
The `-debug` config options are converted at launch into their corresponding
directives for tracing's `EnvFilter`. The new `setlogfilter` RPC method allows
this filter to be reloaded dynamically. The syntax is documented in the
`setlogfilter` help text, as well as here:
https://docs.rs/tracing-subscriber/0.2.7/tracing_subscriber/filter/struct.EnvFilter.html#directives
When `-printtoconsole` is specified, the output now includes timestamps and
ANSI encoding :)
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