JS: readI32 performance on large arrays is very poor in Chrome

`readI32` ( https://github.com/apache/thrift/blob/master/lib/js/src/thrift.js#L1311-L1341 ) performance is extremely poor in Chrome when reading values from large arrays. This is due to the use of Array.shift ( https://github.com/apache/thrift/blob/master/lib/js/src/thrift.js#L1322 ) which seems to have performance issues in V8/Chrome.

As Chrome is a high market share browser I propose either changing this to use reverse/pop or detect the appropriate strategy given array length. I think this is due to how V8 handles large arrays and the heuristics used to determine whether data should be stored in a C-backed array or hash map.

STR:

0) Download or copy the attached ThriftBenchmarker.js - this file has some helper functions to build large arrays, as well as a copy/paste of the current `readI32` as well as one that uses Array.reverse/Array.pop over Array.shift.

1) Run this test in Chrome - you can simply copy/paste the code into the Console.

2) Observe that the tests using Array.shift are quite slow in Chrome:

-- testing long arrays --
shift test: 1084.566162109375ms
pop test: 37.604248046875ms
-- testing long arrays of strings --
shift test: 5189.416015625ms
pop test: 17.9189453125ms
-- testing short arrays --
shift test: 0.06005859375ms
pop test: 0.016845703125ms
-- testing short arrays of strings --
shift test: 0.01611328125ms
pop test: 0.01318359375ms

3) Run this test in Safari (or a non-V8) browser.

4) Observe that the tests using Array.shift are far faster:

-- testing long arrays --
shift test: 19.235ms
pop test: 11.774ms
- testing long arrays of strings --
shift test: 13.087ms
pop test: 12.805ms
- testing short arrays --
shift test: 0.049ms
pop test: 0.054ms
- testing short arrays of strings --
shift test: 0.039ms
pop test: 0.022ms

ThriftBenchmarker.js