Vapor: 4.17.0 Release

This patch was authored by @tancred and released by @tanner0101.

Adds a new method for bootstrapping a custom log handler (#2348).

The new LoggingSystem.bootstrap method accepts a LogHandler factory. The detected logging level (configurable with --log or LOG_LEVEL) is passed to the closure.

LoggingSystem.bootstrap(from: &env) { logLevel in 
    MyLogHandler(logLevel: logLevel)
}

There is also a new helper for parsing Logger.Level from the environment.

let logLevel = Logger.Level.detect(from: &env)