redbiom metadata searches are very slow

[wasade]

Searching for "antibiotics" takes minutes to longer. The same search from the command line (´redbiom search metadata antibiotics´) takes roughly 10s.

[antgonza]

Proposed solution by @wasade:

• cache samples in each context (do independent of web users server side)
• search for samples with metadata
• for each context, get set intersection of samples with what's cached