graphite: max_cache_size to 10,000,000

Set MAX_CACHE_SIZE to 10,000,000, per
 <https://answers.launchpad.net/graphite/+question/133636>.
The first comment is by Graphite's author and is informative.
c.f. "(I used 10 million on machines w/24G memory)"

Change-Id: I884ae8acbf30bce362fcfce216c52ec6d5e6e807