OAK-2025 - Make the cache size in MongoBlobStore configurable

Cache size can be configured via blobCacheSize property for DocumentNodeStoreService

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1617417 13f79535-47bb-0310-9956-ffa450edef68
OAK-2025 - Make the cache size in MongoBlobStore configurable

- Adding testcase for new config
- Updating the docs

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1617418 13f79535-47bb-0310-9956-ffa450edef68