Consider adjusting the default purge size and interval of the WAL

[evenyag]

What type of enhancement is this?

User experience

What does the enhancement do?

The default purge config for the WAL may be too large

greptimedb/src/common/wal/src/config/raft_engine.rs

Lines 53 to 54 in c732016

	purge_threshold: ReadableSize::gb(4),
	purge_interval: Duration::from_secs(600),

The WAL can easily consume more than 4GB of disk space. We may consider adjusting the default config.

Implementation challenges

We may find a proper default value. Users can adjust the value if the default config becomes the bottleneck.

[killme2008]

Let's fix it in v0.11