A few TODOs coming out of #11075

• Add documentation for out of order feature and the config (before v2.39)
• Rename the type WAL to type WL (but the usage retains wal and wbl naming).
• Revisit Head.Appender.Commit() code and try to simplify it.
• Simplify loadWAL and loadWBL in head_wal.go as much as possible (not directly related to the OOO work, since this structure of code existed before)
• make processWBLSamples and processWALSamples look similar
• Add a fast path for write path by checking the last sample Out-of-order post merge cleanup #11329 (comment)
• Some refactor Out-of-order post merge cleanup #11329 (comment)

Few having their own issues:

• Get rid of the EncOOOXOR from the chunk encodings #11835
• Fix the prometheus_tsdb_sample_ooo_delta metric #11836
• Simplify chunks.Meta by removing the OOO fields #11837

cc @jesusvazquez @bwplotka