fix: using options.numBackups instead of options.daysToKeep

[lamweili]

Fixes log4js-node/log4js-node#1080, Fixes log4js-node/log4js-node#1035

It's renamed to numBackups instead for clarity while daysToKeep is still backward compatible.
Usage of daysToKeep will result in process.emitWarning of deprecation.

streamroller/lib/DateRollingFileStream.js

Lines 21 to 25 in 272e5e3

	process.emitWarning(
	"options.daysToKeep is deprecated due the confusion it causes when used " +
	"together with file size rolling. Please use options.numBackups instead.",
	"DeprecationWarning", "StreamRoller0001"
	);