Default core.streamFileThreshold to a larger value

If this value is not configured by the server administrator
performance on larger text files suffers considerably and
Gerrit may grind to a halt and be unable to answer users.

Default to either 25% of the available JVM heap or ~2048m.

Change-Id: Ib428fea3eb126d4ce13c73a5af493f9a86763ac8