box/journal: redesign journal operations

Currently the journal provides only one method -- write,
which implies a callback to trigger upon write completion
(in contrary with 1.10 series where all commits were
processing in synchronous way).

Lets make difference between sync and async writes more
notable: provide journal::write_async method which runs
completion function once entry is written, in turn
journal:write handle transaction in synchronous way.

Redesing notes:

1) The callback for async write set once in journal
   creation. There is no need to carry callback in
   every journal entry. This allows us to save some
   memory;

2) txn_commit and txn_commit_async call txn_rollback
   where appropriate;

3) no need to call journal_entry_complete on sync
   writes anymore;

4) wal_write_in_wal_mode_none is too long, renamed
   to wal_write_none;

5) wal engine use async writes internally but it is
   transparent to callers.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>