Improve on logging

[repeatedly]

1. Improve Log#on_xxx API performance with yield.
2. Old implementation is error prune. API is log.on_info { ... } so error should be thrown when block is not passed.
   I searched the plugin code and no log.on_xxx without block code so it doesn't break existing plugins.
3. Use on_trace API in buffer. trace level is rare on the production so frequent object creation for trace logging is not good.

[mururu]

For 3, my understanding is that we can avoid evaluation for the arguments of log.trace by using log.on_trace. At the same time, we also don't have to create unnecessary Proc object because we use yield even if we use log.on_trace. So it shows better performance, right?

[repeatedly]

Exactly

[mururu]

Thanks. Looks good!