aa/eventlog: fix WAL crash recovery

[mmahut]

Crash recovery for the eventlog didn't work:

• The WAL offset was written big-endian but read back little-endian,
  so recovery seeked to a wrong position.
• The eventlog file was opened with O_APPEND, so the recovery seek did
  nothing and a half-written entry was never overwritten.
• After reopening an existing log, the writer position started at 0
  instead of the end of the file.

Also use write_all() so short writes are not silently dropped, and
serialize the recovered entry the same way as the normal write path.

Fixes: #1566