env_mirror: instruct EnvMirror whether mirrored Envs should be destroyed

env_mirror: instruct EnvMirror whether mirrored Envs should be destroyed

The lifecycle rules for Env are frustrating and undocumented.  Notably,
Env::Default() should *not* be freed, but any Env instances we created
should be.

Explicitly instruct EnvMirror whether to clean up child Env instances.
Default to false so that we do not affect existing callers.

Signed-off-by: Sage Weil <sage@redhat.com>