Remove "object cache" misfeature; add simpler "resource cache"

[danielballan]

The "object cache" was a misfeature because it mixed two distinct resource pools:

(1) Caching chunks of data
(2) Caching file handles

(I admit this should have been immediately clear at the time it was added. This was very early on when we were prototyping a lot of aspects to validate the general idea.)

This PR removes the "object cache" entirely. The only remaining mention is in the service configuration, where object_cache is still accepted, for backward-compatibility, but a warning is emitted that any configuration here has no effect:

$ git grep object_cache
tiled/config.py:        if "object_cache" in config:
tiled/config_schemas/service_configuration.yml:  object_cache:

Then the PR adds in its place a "resource cache" which:

• addresses only file handles (2), not cached data (1)
• involves much less custom code, using cachetools.TLRUCache to do the heavy lifting

Here we can see it in action. The file is only opened while processing the first request. The second request uses a cached file handle.

$ strace -fe open,openat tiled serve directory files/ --public

This is the relevant output. (Some noise related to #663 has been removed for clarity.)

[pid 2870299] openat(AT_FDCWD, "/home/dallan/Repos/bnl/tiled/files/a.tif", O_RDONLY|O_CLOEXEC) = 17
INFO:     127.0.0.1:57168 - "GET /api/v1/array/full/a HTTP/1.1" 200 OK
INFO:     127.0.0.1:57176 - "GET /api/v1/array/full/a HTTP/1.1" 200 OK

[pid 2871270] openat(AT_FDCWD, "/home/dallan/Repos/bnl/tiled/files/stuff.h5", O_RDONLY) = 18
INFO:     127.0.0.1:52242 - "GET /api/v1/array/full/stuff/data HTTP/1.1" 200 OK
INFO:     127.0.0.1:52246 - "GET /api/v1/array/full/stuff/data HTTP/1.1" 200 OK

After waiting for the cache expiry time (default 60 seconds) accessing it again opens the file again, as expected.

The cache parameters are tunable via environment variables, but this is intentionally not exposed through the service configuration or CLI. I think it's possible that we can choose sensible defaults that will not need to be overridden except in debugging scenarios.

Closes #683

[danielballan]

A server side data cache, use case (1), is still of interest, but should be explored in future issues and PRs. The primary goal of this PR is to remove the object cache in preparation for a beta release, and to put in place a way to do basic file handle caching, which we know can be important, to avoid thrashing file handles when serving a burst of parallel requests.

[danielballan]

One more interactive test. If the cache is zeroed out, the file is reopened on each request, as expected:

TILED_RESOURCE_CACHE_MAX_SIZE=0 strace -fe open,openat tiled serve directory files/ --public
...
[pid 2876503] openat(AT_FDCWD, "/home/dallan/Repos/bnl/tiled/files/stuff.h5", O_RDONLY) = 14
INFO:     127.0.0.1:43080 - "GET /api/v1/array/full/stuff/data HTTP/1.1" 200 OK
[pid 2876503] openat(AT_FDCWD, "/home/dallan/Repos/bnl/tiled/files/stuff.h5", O_RDONLY) = 12
INFO:     127.0.0.1:43088 - "GET /api/v1/array/full/stuff/data HTTP/1.1" 200 OK

[genematx]

I really like the mechanics of with_resource_cache, and how it simplifies the code overall. Just a few notes from me; nothing major.