this limit applies to both 32 and 64-bit VMs

In order to make the doc generation for #476 working I had to fix some
errors in existing files. These cover

 - lines which are too long
 - incorrect indentation
 - invalid markup
 - missing javadoc

* Custom data types handler

* Updated doc, changelog, dictionary. Fixed review comment.

* Extended the custom types handler interface

* Fixed test

* Fixed review comments

* Added note on ABI stability of the feature to docs

MVStore: up to 65535 open transactions are now supported. Previously, the limit was at most 65535 transactions between the oldest open and the newest open transaction (which was quite a strange limit).

H2 Console and server mode: SSL is now disabled and TLS is used to protect against the POODLE SSLv3 vulnerability. The system property to disable secure anonymous connections is now "h2.enableAnonymousTLS". The default certificate is still self-signed, so you need to manually install another one if you want to avoid man in the middle attacks.

and which servers that are available. (patch from  Nikolaj Fogh) 

New feature from Davide Cavestro - allow using custom Java object serialization engines on a per-DB basis.