- detailsInMemoryStorageLimit converted to static and tagged as @VisibleForTesting;
 - removed unnecessary use of detailsInMemoryStorageLimit.

…timisation).

…timisation):

 - replace test specific method setExternalStorageDataSource with more restrictive option;
 - corrected testExternalStorageTransition to test the actual transition functionality;

 - storing the encoding increases the complexity of the code with very little benefit;
 - the encoding is computed only once at commit, but copied many times if cached;

…ils.

 - shows that storage transition is more expensive (in time and db size) than the alternative.