Improvements to the matrix cache (#1340)

* Improvements to the matrix cache

- Add a timelimit for the matrix in the cache.
Once the timelimit is reached, the matrix can be removed.

- If a DatasetAccessError occurs, then remove the dataset
from the matrix cache.

Fixes #1322

server/common/default_config.py

@@ -36,6 +36,10 @@ multi_dataset:
     # is evicted from the cache first.
     max_datasets: 5
 
+    # A matrix is automatically removed from the cache after timelimit_s number of seconds.
+    # If timelimit_s is set to None, then there is no time limit.
+    timelimit_s: 30
+
 single_dataset:
   datapath: null
   obs_names: null