Increase default block size

This is to go into 0.8.6 so I'm posting this pull request for completeness.

- increase default size by 50K to 300K
- increase high-priority area from 27K to 30K

#3229 may be a better alternative for master/0.9, as it gets rid of the defaults completely and forces the miners to decide.

Edit: MAX_BLOCK_SIZE_GEN is two times the default size, updated description accordingly