MB-21029: ep_test_apis: make wait_for_stat_XXX functions templated

MB-21029: ep_test_apis: make wait_for_stat_XXX functions templated

There is a range of helper functions to wait for a stat to meet some
criteria (equal, not equal, greater, ...), and currently these are
duplicated for each type (int, string, ...) required.

As the subsequent fix for MB-21029 needs a wait_for_stat_to_change for
the std::string type, genericize the current wait_for_stat function so
it can be used with any type.

Change-Id: I681218d31c4dcd1ef8b34de225efd13e99bbf8db
Reviewed-on: http://review.couchbase.org/67942
Reviewed-by: Manu Dhundi <manu@couchbase.com>
Tested-by: buildbot <build@couchbase.com>