Investigate setting `-XX:MaxRAMPercentage` to >25% of system memory

[taylor.smock]

The flag was added during the Java 8 update cycle (but not as part of the initial Java 8 release).

The default is 25% of system memory; we can probably safely use up to 75% of system memory.

We may also want to look into setting the minimum memory (Xms) to something semi-sane.

Possible minimums for Xms:

• 99mb: Clean JOSM install after starting
• 136mb: JOSM with minimal amount of data loaded (1129 nodes, 414 ways, 13 relations)
• 161mb: JOSM with minimal amount of data loaded + background layer

I'd probably round up to the nearest power of 2 though, so 128mb or 256mb.