Increase Maximum String value limit in 2.15 to 5 megs (from 1M in 2.15.0-rc1; unlimited before)

[cowtowncoder]

As per concerns from some developers, I think I'd like to tune one of processing limits: that of maximum String value length. 2.15.0-rc1 imposed limit of 1_000_000 characters.
Since this is unlikely attack vector, but possibly something that affects certain binary data embedding (base64) use cases, I think we may want to start with higher limit.

I suggest 5 megs, to keep contiguous JVM heap usage at 10 megs.
This is very much an arbitrary limit; if anyone has ideas of better limit, please add comments.

Note: as an attack vector this seems like a low-risk one: attacker must provide all the data and cannot force pre-allocation (reads are done with incremental buffer allocation).