Export float/double values in decimal notation. Fixes #1781.

This aligns the representation of double values with the format used
in the browser, as well as producing a notation that can be feed back
to Cypher.
Adds thread info to relevant exception messages
Merge pull request #1792 from jotomo/1781-exporter-double-to-string

Export float/double values in decimal notation. Fixes #1781.
Master generates and propagates an epoch

that slaves must use for all further communication with it, or fail with
InvalidEpochException. This is to prevent the scenario where one part of a
transaction or commit communicates with one master, there's a master
switch where the new master instance uses the same host and port as the old one.
If a transaction would continue through such a scenario then unexpected
things and bad things might happen.
Removes synchronized block around commit

that was introduced by 9b7384ad2f69d9c3a768fe59480a44230214cb81.
Since then these committs: a88607cbe23d8146d8540d62061276866d2b1d02,
b76f8e34f7184e1457e37f6da6137bfbde79c3e5 have addressed the underlying
issue so that the synchronization is no longer needed.
Refactor master -> slave pushing to its own class.

 o Moves half the master -> slave push logic to its own class
 o Background slave pusher now uses the Neo Scheduler for its threads
 o Fixes a bug where the previous implementation didn't take into account
   that the queue of pushes sometimes does not accept another item,
   leading to the committing thread waiting for a callback that never
   gets called.

 o Remaining: There are still two pools of threads used for the pushing,
              it should all be consolidated into one service.
 o Remaining: The current implementation still leaks threads in a long
              running cluster. It maps threads to slaves, but does not
              do any cleanup if a slave disconnects. It would probably
              be better to consolidate all pushing into a single background
              thread.
 o Remaining: The push protocol is still master-[notify]->slave-[pull]->master
              it should be master-[push]->slave