Add $wgDebugDumpSqlLength

When $wgDebugDumpSql is set, the logged queries are truncated to 500
bytes. This is often not enough if you're wanting to actually debug
the queries instead of just seeing that queries were executed.

Change-Id: Iad3abd20c11d647834aa5546a1a9c82916c6519f
Merge "Add $wgDebugDumpSqlLength"