Optimize the performance of the nREPL message log

[bbatsov]

As discussed in #1758 the nREPL message logging is pretty slow (especially when big data structures are involved) and enabling logging causes constant freezes.

To alleviate the problem logging was disabled by default, but we still need to analyze and optimize the problematic code (nrepl--pp seems to be the main culprit here).

//cc @vspinu

[vspinu]

The issues seem to be coming from pp-to-string. It shouldn't be hard to get rid of it.

[bbatsov]

@vspinu Can you look into this?

[vspinu]

Sure. I will have a look over the weekend.

[bbatsov]

@vspinu Thanks!