relay: Reduce compression level when streaming
Turns out that most of the time, relays will stream back directory content on the highest compression level which means a lot of CPU for the best compression ratio.
However, in the case of authorities, when under DDoS from clients fetching non stop directory documents, it leads to a gigantic amount of CPU time being used on compressing requests that can not be pre-compresssed.
We propose to reduce the compression level, in case of streaming, to the lowest level. Considering that authorities, using Zstd, will see a significant reduction in CPU time but also will still gain on using the highest compression level that Zlib offers.
See https://docs.google.com/spreadsheets/d/1devQlUOzMPStqUl9mPawFWP99xSsRM8xWv7DNcqjFdo/edit#gid=0
Pre-compressed data will still use the current values, only streaming data will be downgraded to lowest ratio.