LU-12577 llog: protect partial updates from readers

LU-12577 llog: protect partial updates from readers

llog_osd_write_rec() adds a record in few steps: the header is
updated first, then the record itself is appended. per-loghandle
semaphore is used, but remote readers allocate a new separate
loghandle for every access (header reading, blocks), the the
readers can't use loghandle's semaphore to avoid accessing partial
updates. use object-based locking [censored] to serialize the writer
vs the readers.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ie4e4d4a1e9a6fcdea9fcca7d80b0da920e786424