Fixes #31 Memory leak in 0.9.12

[d3m3vilurr]

Simple test case:

var immediately = function(cb) {
    sync.fiber(function() {
        setImmediate(sync.defer());
        sync.await();
    }, cb);
};

function test(m, cb) {
    if (m-- === 0) return cb();
    immediately(function(err) {
        if (err) return cb(err);
        if (m % 1000 === 0) {
            console.log(m + ": " + JSON.stringify(process.memoryUsage()));
        }
        test(m, cb);
    });
}

test(100000000, function(err) {
        if (err) throw err;
        console.log("done");
});

Before:
99888000: {"rss":114892800,"heapTotal":81994496,"heapUsed":58052656}
99887000: {"rss":115163136,"heapTotal":81994496,"heapUsed":59327264}
99886000: {"rss":115433472,"heapTotal":83026432,"heapUsed":60601944}
99885000: {"rss":115703808,"heapTotal":83026432,"heapUsed":61877016}
99884000: {"rss":115974144,"heapTotal":83026432,"heapUsed":63152072}
99883000: {"rss":120840192,"heapTotal":87154176,"heapUsed":56497944}
99882000: {"rss":121110528,"heapTotal":87154176,"heapUsed":57772688}

After:
99687000: {"rss":74567680,"heapTotal":60323840,"heapUsed":36945504}
99686000: {"rss":74567680,"heapTotal":60323840,"heapUsed":38211632}
99685000: {"rss":74510336,"heapTotal":59291904,"heapUsed":16835640}
99684000: {"rss":74510336,"heapTotal":59291904,"heapUsed":18102288}
99683000: {"rss":74510336,"heapTotal":59291904,"heapUsed":19368336}
99682000: {"rss":74510336,"heapTotal":59291904,"heapUsed":20634792}
99681000: {"rss":74510336,"heapTotal":59291904,"heapUsed":21900840}
99680000: {"rss":74510336,"heapTotal":59291904,"heapUsed":23166888}
99679000: {"rss":74510336,"heapTotal":59291904,"heapUsed":24432936}

[bradvogel]

We tested this in production (with very high volume) today and it appears to work well. Thanks!

[bradvogel]

Please bump npm version also

[bradvogel]

@alexeyPetrushin can you merge this? This regression is very severe and probably affecting many others.

[al6x]

Hi, sorry for delay, moved to a new house and was couple of days without internet...
Thanks for fix, will merge it immediately.

[al6x]

Released as 0.9.14