Move flags to ReactorStmtCtx

We have a bunch of bitwise flags we are using for various
inference details. Unfortunately them being placed in
StatementContextBase and ReactorStmtCtx layout result in 8 bytes
being lost on 64bit w\o compressed oops.

Move the flags and their methods to ReactorStmtCtx, reducing
InferredStatementContext footprint from 160 bytes to 152 bytes
(i.e. 5%).

JIRA: YANGTOOLS-1184
Change-Id: I0c9787795fc67c1e979132f9244f06a7a7e3dc3e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>