Avoid marking map blocks dirty upon deserialization.

[lhofhansl]

Improves #10566 (but does not fix it completely)
@hecktest

As is just de-serializing a block from disk marks it dirty, which makes no sense.
That looks like an unintended side-effect of setting the blocks timestamp.

Note that active blocks are still marked dirty every 60s, but is a subset of all blocks.

[hecktest]

Does this also reduce the lag from map saves?

[sfan5]

Through extensive thinking I have concluded that this is a good idea.

[lhofhansl]

@sfan5 :)

@hecktest Unfortunately not. Mapsave cause lag because (1) it always saves the whole block even just one attribute changed, (2) compression is slow'ish, and perhaps most importantly (3) it's done while holding the global environment lock (so no emerges, no blocks send or received to the client, no active blocks or objects, etc, etc can happen at the same time)