Emphasize context managers when working with Core

Emphasize context managers when working with Core

Prep the main documentation for the changes coming up in 1.4
by first removing references to engine.execute() outside
of the "connectionless" section, and using context managers
in all cases.  For features that have always been confusing
and are going away, add a note that this feature will
be going away.

Change-Id: I94583444734e36f5432c32cb37f88a9fad15634a
(cherry picked from commit 376708f4a4958bf2559c14900c52aa6fc7fd05b3)