Logging, docs and style improvements.

No behavior changes.
Fix Reactive*ViewSource problems with ReloadData().

If a data source changes too many times, it is possible to break
a UI{Table,Collection}View.  Please see the docs of
lastCallToNumberOfSections introduced by this commit itself,
which fixes this problem.

This commit also adds hooks to Changing observables, thus releasing
bindings earlier than before.  I do not think that this is necessary
for correctness, but it does not seem to harm and may be beneficial in
avoiding needless work in some scenarios.
Less Timestamped mess.
Avoid DateTime in timestamps, use UInt64 instead.
Style fixups
Merge pull request #433 from reactiveui/reloaddata-sucks

Fix Reactive*ViewSource problems with ReloadData().