Show big lake names earlier

[kocio-pl]

I think big lake name labels should be visible on z7+ (start of the midzoom) - z10+ is much too late:

http://www.openstreetmap.org/#map=10/43.8325/-77.6795

[kocio-pl]

It makes sense for me to start even from z6, because it's a small, limited change for low zoom and big lakes start showing with a proper size to have labels.

[kocio-pl]

For biggest lakes (filtered only water=lake tags to make database import time manageable) it seems to be working great:

z6
Great Lakes

Caspian Sea + Aral Sea

However there are some strange label placement issues, though I have only changed zoom from z10+ to z6+ in my draft code:

1. Sometimes it's different with filtered data than on current map:
   Ladoga Lake
   

2. Sometimes it's already there, it just would start showing earlier:
   Lake Yessey
   

3. Sometimes it's hard to say yet, because I couldn't find a label on the current map:
   Lake Victoria
   

Any ideas what could it be?

[kocio-pl]

At least it looks like dataset was responsible for inconsistency - full data from Ladoga Lake area give the labels placed as expected:

z6

[kocio-pl]

This is somewhat related to #1604, because starting zoom level for labels depends (to some extent) on starting zoom level for showing lakes.

[kocio-pl]

Just a note: the problem with label placement is probably described in #1465.

[kocio-pl]

Resolved by #2873.