Reverts frame_by_id behaviour

[ChristianSteffens]

This relates to https://github.com/ebroecker/canmatrix/pull/825.

While speeding up frame_by_id is useful, it changes the original behavior: Our environment expects a fresh, up-to-date frame each call, whereas the cache may return outdated data (e.g. after renaming a frame).

To stay consistent with other APIs, I’d suggest keeping two variants:

• get_frame_by_attribute: cache-based
• frame_by_attribute: always searches

Alternatively, the cache could be reset whenever a frame attribute changes, but that would require much more intrusive changes. I’d prefer keeping the PR as minimally invasive as possible.

[ebroecker]

should be merged with #884