Avro, Date/Time handling: feedback/review for #283 to be added for 2.13

[Tatu Saloranta]

So, since I know there have been many attempts to improve Date/Time
handling for Avro module -- and since Date/Time handling in general as
well as in Java in particular is ... complicated -- I thought I'd ask
for help in reviewing this neat new PR, planned for inclusion in 2.13:

https://github.com/FasterXML/jackson-dataformats-binary/pull/283

the basic idea is that instead of solving more general Logical Types
problem (something I eventually want to tackle), there's an optional
Module to add to AvroMapper, which would allow more efficient
serialization of a subset of Java 8 date/time values into `long`s and
so forth; as well as generating better Avro schemas with datatype
awareness.

I think the main concern I have is with "Local" date/time types: these
have been proven to be VERY confusing to most users, developers; and
it seems that some libraries/frameworks have added to confusion as
well -- both in documentation and in implementations.

So: before merging things in, it'd be great to get more eyes to go
through these changes. This is not a huge PR, fortunately, so I hope
someone with Date/Time-fu has a bit of time to spare for reviewing it.
:)

-+ Tatu +-