Java's zip FileSystem is rather memory-hungry, as it maintains the
full central directory in memory, among other internal state. In the
headless server case, we never actually need to serve a UI from the
war, so it's not necessary to even load the zip FileSystem.
Maintain a synchronized map of zip FileSystems in GerritLauncher,
since each one still needs to be a singleton, but we don't want to
eagerly load it just because we wanted to look up the war location.
Change-Id: Id186964b2a197a8a0e874f86c72c21e06fc1388f
f3ff4db9c3