OAK-1815: Don't create default (Scheduled)Executor when respective instance is supplied
Create executor service instances only when needed

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1594205 13f79535-47bb-0310-9956-ffa450edef68
OAK-1812

- catch attempts to write blobs that are too long
- add tests
- increase default blob size in DB2

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1594207 13f79535-47bb-0310-9956-ffa450edef68