0.9.10 Per message substitution of ${ctx:} lookups

All environment, system etc. variable references are be replaced by their values during initialisation of the appender. This release introduces support for ctx (MDC) parameter lookup done in a dynamic way - per log message.

This functionality uses internal log4j2 classes so logic is the same as in Lo4j2. The implementation is not optimal and allocates objects for each message. This will be addressed in a future release.