Update how REPLServer uses contexts

[weaver]

Hi, I was trying to subclass REPLServer to not share a context across instances. It turns out there's no way to do this conveniently because the current REPL implementation refers to a global context variable in several places. This patch does the following:

• Always use this.context or self.context instead of context.
• Move resetContext to REPLServer.createContext.
• Add REPLServer.resetContext, memoize context here.
• Memoize exports.repl in start instead of REPLServer constructor.

My use case is a local "administration" REPL for a web service. If several people are connected to the REPL, I don't want their context shared because they may clobber each others bindings by defining variables with the same name or using ".clear".

With this patch, the following works as expected (each repl has a unique local context):

util.inherits(REPL, repl.REPLServer);
function REPL(prompt, stream) {
  repl.REPLServer.call(this, prompt, stream);
}

REPL.prototype.resetContext = function() {
  this.context = this.createContext();
};

[ry]

Thanks Ben. Looks good. landed in master

[ry]

landed in d63a551