Add support to have IsCached and UseSeparateThreadToWrite properties set to True at the same time

[jachguate]

Actually, if both are set to true, the cache mechanism doesn't work, but the messages are directly written using the auxiliary trheads.

When WriteCachedLog is called, the CACHED LOG BEGIN/END messages are written in the context of the calling thread and not using the writer's own threads.

When both are set to true, the messages have to be cached and when WriteCachedLog is called the threads created for that purpose have to do the actual work.