MB-9192: fix leak caused by cells capturing callstack at creation

Here's how it ended up leaking memory on analytics.js

There's "infinite" future implemented by createSamplesFuture that
implements stats polling loop. In order to fetch actual data from
server it's using getCPS function defined nearby that works through
cell's iocenter facility but behaves like traditional callback-style
ajax call. In order to do it it creates empty cell for every request.

Now recall that without this commit constructing cell captured full
stack trace at time of creation for debugging purposes. It turns out
Chrome debugger finally gives you _full_ access to every stack trace
element. Including full access to it's environment (all
variables). That makes all stack fraces "live" for purpose of gc.

Normally that would not be a problem because cell itself would be
shortlived. It's used for just single request after all and is only
referred from async variable in getCPS. _But_ subtlety that keeps it
alive is that getCPS callback typically does another getCPS request
(via samples restorer and doGet function). So cell of second getCPS
request will have getCPS's inner function on it's callstack. That
inner function will keep environment of outer getCPS in it's
closure. Thus keeping outer getCPS' async variable and it's cell
"live". Next getCPS will keep second getCPS instance alive etc.

That chain of closures keeps full results of all previous samples (via
lastValue argument of samples restorer which is part of preserved call
stacks). So leak is quite big.

It could be fixed in a number of ways. For example:

* make sure that "trampoline" that calls getCPS callback does not see
  getCPS environment

* clean createdAt property of cell used in getCPS

* do not do next getCPS call immediately from callback of previous
  getCPS (i.e. via _.defer or setTimeout)

But most simple fix is to stop capturing createdAt property of cells
which is only useful for debugging and is otherwise useless and
dangerous overhead in production.

Change-Id: I2ecf7544b9de2d19309d32991f1d3e845251e855
Reviewed-on: http://review.couchbase.org/29349
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Pavel Blagodov <stochmail@gmail.com>
Tested-by: Pavel Blagodov <stochmail@gmail.com>