Switch to Java 8

[Don-vip]

This ticket is the successor of #8465 :)

Like for Java 7 two years ago, Oracle released their final versions of Java 7 (7u79 and 7u80), and the same auto-update feature for Windows and OSX users. We already have a significant majority of users who run JOSM with Java 8:

All users:

Java Main Version --> 6 (598,  4.3%) 7 (5128, 37.3%) 8 (8009, 58.2%) 9 (22,  0.2%)

Recent versions (>=7001):

Java Main Version --> 6 (37,  0.3%) 7 (4370, 35.7%) 8 (7818, 63.8%) 9 (22,  0.2%)
OS:  FreeBSD (2,  0.0%) Linux (2749, 22.6%) Mac (911,  7.5%) OpenBSD (1,  0.0%) SunOS (3,  0.0%) Windows (8500, 69.9%)

Java 7 users:

OS:  FreeBSD (2,  0.0%) Linux (2394, 49.5%) Mac (129,  2.7%) OpenBSD (3,  0.1%) SunOS (3,  0.1%) Windows (2310, 47.7%)

Java 8 users:

OS:  Linux (567,  7.1%) Mac (786,  9.9%) Windows (6613, 83.0%)

Linux is clearly behind. Main distributions of Java 7 users are:

O        251 ( 5.2%) Linux Debian GNU/Linux 8.0 (jessie)
O        157 ( 3.2%) Linux Mint 17.1 Rebecca
O        102 ( 2.1%) Linux Ubuntu 14.04.1 LTS
O        809 (16.7%) Linux Ubuntu 14.04.2 LTS
O        154 ( 3.2%) Linux Ubuntu 14.10
O        222 ( 4.6%) Linux Ubuntu 15.04

Java 8 is not yet available in Debian Jessie or Ubuntu LTS, but it will be ​released soon in ​Jessie backports then ​Trusty backports.

The transition will be easier than Java 7:

• Java 8 is available and stable on all platforms
• There's no need this time to build a new package for OSX
• JOSM is ​built and tested daily with Java 8 for quite some time. It works fine and there's even a large performance gain in rendering: see ​Java 7 results vs ​Java 8 results

So, what's new?

• ​What's New in JDK 8
• ​Incompatibilities between Java SE 8 and Java SE 7
• ​Known Issues for JDK 8

Other useful links:

• ​https://leanpub.com/whatsnewinjava8/read
• ​https://www.techempower.com/blog/2013/03/26/everything-about-java-8/

Proposed plan:

1. Restore informative message "JOSM will soon no longer work with this version of Java..." on server side
2. Ask OSX and Windows users to switch (revert/update this and this)
3. Wait for Jessie and Trusty backports
4. Ask Linux users to switch (revert/update of this)
5. Switch about two months after that