v2: Logging! (#2831)

* logging: Initial implementation

* logging: More encoder formats, better defaults

* logging: Fix repetition bug with FilterEncoder; add more presets

* logging: DiscardWriter; delete or no-op logs that discard their output

* logging: Add http.handlers.log module; enhance Replacer methods

The Replacer interface has new methods to customize how to handle empty
or unrecognized placeholders. Closes #2815.

* logging: Overhaul HTTP logging, fix bugs, improve filtering, etc.

* logging: General cleanup, begin transitioning to using new loggers

* Fixes after merge conflict

context.go

@@ -22,6 +22,7 @@ import (
 	"reflect"
 
 	"github.com/mholt/certmagic"
+	"go.uber.org/zap"
 )
 
 // Context is a type which defines the lifetime of modules that
@@ -206,3 +207,8 @@ func (ctx Context) App(name string) (interface{}, error) {
 func (ctx Context) Storage() certmagic.Storage {
 	return ctx.cfg.storage
 }
+
+// Logger returns a logger that can be used by mod.
+func (ctx Context) Logger(mod Module) *zap.Logger {
+	return ctx.cfg.Logging.Logger(mod)
+}