For better ergonomics. I always found it weird that we used UUID to
actually mean a tenant or timeline ID. It worked because it happened
to have the same length, 16 bytes, but it was hacky.
Resolves#1889.
This PR adds new tests to measure the WAL backpressure's performance under different workloads.
## Changes
- add new performance tests in `test_wal_backpressure.py`
- allow safekeeper's fsync to be configurable when running tests