Allow the raw compression function to fail, in which case we write the
original data as a single page.

Don't accumulate more data unless the raw compression function requests
it (by returning EAGAIN), or if we're stuck because the raw compression
function consumed all of the data and there's more data so we may need
a key for the next block.

Reference #1007, #1008.
Merge pull request #1013 from wiredtiger/compression-can-fail

Handle raw compression giving up.