H2 database by default uses up to the half of the amount of memory
available for the virtual machine (Runtime.getRuntime().maxMemory())
as described in [1]. In some cases this could contribute to an
out-of-memory error in Gerrit.
Introduce a new configuration parameter cache.h2CacheSize to limit
the H2 memory cache size. In this change this parameter is applied
to all persistent caches.
[1] http://www.h2database.com/html/features.html#cache_settings
Change-Id: I0217d4114f528275ca4094e0f3414ef23af80677
f313457af5