fix: make logger work while loading config file

[SteveLauC]

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• The code passes tests (cargo test)
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What does this PR do

1. Make the logger work while loading the configuration file

   A few days ago, I noticed that the configuration file was no longer printed in verbose mode, and found that our logger will ONLY be initialized after loading the config file, this behavior was introduced in Allow setting misc.log_filters in config.toml #552, and this PR fixes it by setting our logger in 2 phases:

   1. Set up it with the log level (filter directives) specified in the CLI option

      let reload_handle = install_tracing(&opt.tracing_filter_directives())?;

   2. Update it with the log levels specified in the CLI option and the config file

      update_tracing(&reload_handle, &config.tracing_filter_directives())?;

      For more details, see the code comments

2. Replace the default log level setting with a const variable DEFAULT_LOG_LEVEL

3. Style change, import tracing::<log_macro!> when using it