Increase ApiQueryMessageCollection maximum limit

The performance is not dominated by the number of messages to fetch,
but instead of constant overhead cost which varies by the group, as
can be seen in the following tests using ab for query like the one
below - a query made by Special:Translate on normal view.
/w/api.php?action=query&format=json&list=messagecollection&mcgroup=XX&
mclanguage=fi&mclimit=YYYY&mcfilter=!ignored%7C!optional%7C!translated
&mcprop=definition%7Ctranslation%7Ctags%7Cproperties

Change-Id: Id0f4dc3255e4276df6cfc6a70c48f4531c64bc97
              min  mean[+/-sd] median   max    group   limit
Total:       5486 5577  57.7   5600    5633 mediawiki    100
Total:       5691 5993 285.2   5966    6462 mediawiki    500
Total:       7481 7802 477.9   7631    8649 mediawiki   5000
Total:       1286 1342  33.3   1355    1372      core    100
Total:       1267 1319  36.5   1338    1356      core    500
Total:       1270 1317  54.6   1316    1401      core   5000