journaler: consolidate journal entry buffers

the mds' journal writes to the objecter were sending thousands of buffer
segments, and eating up the accelio messenger's resources

Journaler::append_entry() now uses bufferlist::append() to consolidate
the small buffers into the bufferlist's page-sized append_buffer

Signed-off-by: Casey Bodley <casey@linuxbox.com>
journaler: fix for split case of append_entry

Signed-off-by: Casey Bodley <casey@linuxbox.com>