Replace os_cache_max configuration and behavior with calls to: (1)
discard any page read from the system cache, (2) immediately schedule
all writes to the disk, and periodically discard any pages written
from the system cache.  In all cases, ignore if O_DIRECT is configured.
Add the new map file.
Don't mmap(2) the file if it's configured for direct I/O, the Linux
man pages say it's a bad idea.
Merge branch 'develop' into fadvise
Re-work posix_fadvise, sync_file_range buffer cache changes.

Put per-object os_cache_max configuration back in, but implement it in
the block manager layer.  Disallow if direct_io is configured, or if the
system doesn't support posix_fadvise.

Add per-object os_write_schedule, if configured immediately schedule
any writes.  Disallow if direct_io is configured, or if the system
doesn't support sync_file_range.
Turn of file-mapping if the application configured a cache-size maximum.
Change os_write_schedule to be os_cache_dirty_max, changing a boolean
(schedule writes for each buffer after it's written) into a value
(schedule writes for all dirty buffers after enough bytes have been
written).
Don't increment by offset, increment by size.
Merge branch 'develop' into fadvise
Merge pull request #472 from wiredtiger/fadvise

Add configurations to minimize the impact of reads and writes on the filesystem cache.