content-sqlite: treat ENOSPC as a transient condition

[garlick]

Problem: when the file system containing statedir on rank 0 fills up, content-sqlite propagates errors back to the content cache which may not handle it well (#5978).

Potentially content-sqlite itself could navigate this situation without ever returning errors to the content cache.

[garlick]

Some investigation is needed on if/how sqlite can recover from this, starting with recreating the situation a few times and cataloging the failure modes.

Other random thoughts

• We could try vendoring sqlite to ensure consistent behavior while probing. Cherry pick from my job-sql branch 7314c710f3d71f54774c12aa39e29494ae9b51b2 and 57d0bad06bc80e6a53b1d4bed24297f882af8e31. I don't think it's out of the question to vendor it in production if older versions are found to have different failure characteristics.
• content-files may be an easy back end to prototype a recovery behavior and write tests for. E.g. when it gets ENOSPC on write, unlink the object and queue store requests until there is free space.
• if #5978 is reproduced during this we should run it down
• I believe if the content backend stops answering store requests, the rank 0 content cache will grow and potentially many store RPC futures will pile up. We might want a way to tell the content cache to pause the back end and/or to throttle repsonses to store requests that are contributing to memory growth.