RAP-258 Refactor MSWriter process function

The process function first updates its internal buffer and then all process functions use this internal buffer for the processing. These functions don't use the internal buffer directly instead they use the buffer provided as function argument. This is a preparation to offload the writing to a separate thread. When that happens both the main and the write thread need to have their own buffer to prevent data races.

Adds a few additional timers to get better insight where time in the writing process is spend.