The `SessionManager` in `exec_command` owns a number of
`ExecCommandSession` objects where `ExecCommandSession` has a
non-trivial implementation of `Drop`, so we want to be able to drop an
individual `SessionManager` to help ensure things get cleaned up in a
timely fashion. To that end, we should have one `SessionManager` per
session rather than one global one for the lifetime of the CLI process.
