Fix statement prerequisites and materialization

Fix statement prerequisites and materialization

We are firing onStatementAdded() when copying statements, which means
that:
a) a new inference requirement may be created during inferred
   statemenent materialization
b) the inference requirement may be immediately available in effective
   model

In order to deal with this, ModifierImpl has to always go through a
bootstrap, so that it prerequisites only trigger once an action is
registered.

Furthermore InferredStatementContext needs always instantiate the map
for partial instantiations so as to deal with namespace-driven
instantiations happening while full instantiation is going on.

JIRA: YANGTOOLS-1434
Change-Id: I86dae587c1fe5804cd983c194903e1975f257408
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>