Made the cache support specifying the block size, breaking the BlobTest.
Made evicter_t not be a friend of page_t.  Now it is a frenemy.
Fixed errors in the BlobTest (bugs in prepend related to specifying the ser buf size -- there were no bugs applicable to the rdb protocol).
driver/python: fix import_format error

I tried to import a csv file with import subcommand, however, it failed
due to import_format was set to "json" by default if --format is not
given:

    $ rethinkdb import -f x.csv --table test.csv_example
    [                                        ]   0%
    0 rows imported in 1 table
    Traceback: [('/usr/lib64/python2.7/site-packages/rethinkdb/_import.py', 458,....)]
    RuntimeError: Error: JSON format not recognized - file does not begin with an object or array
    In file: /path/to/x.csv
    Errors occurred during import

I guess we can do better by trying to get the proper format from the
filename, and then set it to "json" if failed.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Merge branch 'next' into sam_alt_block_size
Made page_t::hypothetical_memory_usage be const.
Merge pull request #2097 from Aleaxander/next

driver/python: fix import_format error
Merge branch 'sam_alt_block_size' into next
Merge branch 'next' into sam_issue_97

Conflicts:
	src/buffer_cache/alt/evicter.cc
	src/buffer_cache/alt/evicter.hpp
	src/buffer_cache/alt/page_cache.cc