* first refactoring to add connection pool

* pool added

* removed usage of KeepAliveSession which solely used as performance benefit favoring perf over complexity.

now with pool, there is no perf advantage by KeepAliveSession. it should be only used when one wants to make connection stateful.

* change remote call signature since solution is now set automatically

* fixed test failures

* made some constant an options. and made pool off by default

handle pool shutdown better

* now turn it on by default

* PR feedbacks

* address more PR feedbacks

* more comments

* renamed ConnectionPool to ConnectionManager. people seems confused by name too much. pool is just one of thing it does, it owns connection management in general.

* PR feedbacks

* change Contract.Fail to Contract.Require so that it doesn't crash VS when OOP is killed explicitly by users.

* PR feedbacks

* address feedbacks

* put finalizer under debug flag