BUG-7052: reduce StatementContextBase proliferation even more
This splits up the mutable and immutable views of declared/effective
statements so we do not access mutable state when all we need is
immutable view.
The mutable view now has only StmtContext.Mutable, hence users get
more isolation from the reactor implementation.
Change-Id: I3e77b3837beea533590cad681c1895e047c9a2a6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
12880a5876c644243ba46ce8c674f29a7d0c773a)