collect: creating buffer pool for sending datapoints

[captncraig]

This is a minor optimization that should reduce gc pressure in the collect package.

Tested with a simple app that sends batches of 300 data points to a local black hole server every 30ms. Monitored with expvarmon

super unscientific data:

Before:

After:

Notice difference in HeapAlloc and Alloc. gc time is pretty much negligible at this scale, but its always good to throw away less stuff.

[gbrayut]

tested and LGTM