This makes sure the file descriptors are closed in the right thread.
Before on NuttX, when stopping the logger, orb unsubscribe failed due to
this.

We always write larger chunks (orb messages) to the buffer, so the buffer
needs to be larger than the minimum write chunk

write_changed_parameters() also takes the lock and thus would deadlock
otherwise.

This is required for the logger, we just manually add the padding bytes
what would otherwise be done by the compiler. Additionally we reorder
the fields by type, so that padding is only necessary for nested types.

it's not used in the logger, so don't store it there. It is accessed via
LogWriter::write.
This also makes sure the buffer size is >= _min_write_chunk and handles
allocation failure properly.