Add a test for simultaneous reads/writes

[lachenmayer]

This test works fine in random-access-file, but breaks in my project https://github.com/lachenmayer/random-access-key-value because I check the database to read the "current" value, and sometimes the previous write has not yet completed.

I noticed this when trying to use random-access-key-value with a hypertrie as storage backend (as in this gist). The root cause is that hypercore writes to some tree location 3, and then writes the new root to tree location 2 instantly, apparently without waiting for completion from the first write.