(SERVER-150) Refactor `prime-pool!` to be more re-usable

The previous implementation of `prime-pool!` required a
`PoolContext` object as input.  This meant that the
function could not be used to prime a pool that was not
yet part of a `PoolContext`.

This commit refactors the function so that it operates on
a `PoolStateContainer` instead; this will allow us to
initialize a pool before putting it into the `PoolContext`.