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.
Made bytes_loaded_counter_ be an int64_t, not the bad intptr_t.  Made get_bytes_loaded return a signed int64_t instead of a uint64_t.
Made update_memory_limit not completely wipe out a timeslice of bytes_loaded_counter_ increments.
Made clearing of per-thread access_count values use subtraction instead of setting to zero.
Made the evicter support decrementing bytes_loaded_counter_ when a page is removed.
Moved get_clamped_bytes_loaded to the .cc file.
Merge pull request #2097 from Aleaxander/next

driver/python: fix import_format error
Fixed comment for bytes_loaded_counter_.
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
Merge branch 'sam_issue_97' into sam_issue_97_uncount_deleted
Fixed compilation errors with merge and hypothetical_memory_usage.
Renamed variable from ser_buf_change to in_memory_buf_size.
Renamed variable from in_memory_buf_size to in_memory_buf_change.
adding evicter size tracking for when page is unevicted