Indexes containing dates may not be updatable or deletable

[dimitribouniol]

The Date in the index is encoded with milliseconds precision, but this may not match the in memory representation nor the persisted representation. For not, don't use indexes involving floats.

To fix, a) have a stable index representation as in #176, and b) make sure primary record points back to indexed entries. (This might be catastrophic for entries whose identifier is a float…)