Log4j-web fails when used with anything that stores into the external context field

Log4j-web stores a reference to the servletcontext into the external context field of the LoggerContext. If another component, such as Spring Boot, stores into that same field then the servlet context reference will be lost. Support for a map of references was added in an earlier release to support Spring Boot. The servlet context should be stored into its own entry in the map rather than the single external context field.