Allow for responses larger than 999999 characters

The protocol officially allows for 6-digit length headers, but in practice
some SlimServer implementations (like the Java one) may emit larger results.
Of course that's likely to cause problems elsewhere, but AFAICS there's no
reason we shouldn't be lenient about that.

Roughly this patch was earlier accepted as 0bbd3c4646e32c2891210aaa18234646a22dfd01
and then reverted in e98714c41f7a3035f9871a687a209aa126d26374, I don't remember
why...
Accept large instructions from the Slim client
Merge pull request #504 from raboof/acceptLargeSlimMessages

Accept large slim messages