cache() with capacity hint overload

[benjchristensen]

Provide a cache(int capacity) overload that acts as a hint for memory allocation.

Ideally it should also allow for an optimization with ReplaySubject when the capacity is 1 so it can use a simple object reference to store the single value and not allocate an array. If more than 1 is received then it would obviously have to switch to using an array, but there are many case, such as Hystrix that would benefit from this change.

[gliptak]

I'm not sure how to approach changingUnboundedReplayState/ReplayState as both complete() and error() methods will "force" to change from single value to array ...

[gliptak]

I missed the discussion with @jbripley #1338

[benjchristensen]

This is done good enough for now as of #1393 which was merged.