Set the memory_page_max of LSM chunks to double the chunk size, so that application threads don't try forced eviction as soon as the "no eviction" flag is cleared by the LSM worker thread.

refs #859 (discovered during testing)
Note that LSM's `chunk_size` setting overrides `memory_page_max`.
Merge pull request #861 from wiredtiger/lsm-memory-page-max

Set the memory_page_max of LSM chunks to double the chunk size.