Stalled backend for large cell data payloads

[jimmymathews]

When the cell-data API endpoint is queried for a sample with a large number of cells (e.g. 700k), the query seems to be forwarded correctly to the cells service, which then correctly finds a cached version (if it exists), but the API service does not seem to be receiving the payloads from the cells service. Possibly there are cluster-internal limitations that stall such transfers.

For this issue, diagnose exactly how the stall happens and try to fix it.

[jimmymathews]

A quick fix was implemented in PR #315, but this is meant to be just temporary.