issue 39: make clear what "plog" is. solve name ambiguity.

plog_dev is split into plog_segs
plog_segs contains one or more plogs
plog contains metadata(plog_meta) and data(1-8 sectors)
and plog write is always atomic.

RENAMES:
plog_size -> plog_seg_size
and so on

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
RENAME: origin_dev -> backing_dev

calling HDD backing device is more preferred
origin device is too general not for caching specifically.

my test scripts entirely use the word BACKING but the code uses
origin is inconsistent. this patch also fix this problem.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>