Improvements to imposter tables: (1) They are read-only unless writable-schema

Improvements to imposter tables:  (1) They are read-only unless writable-schema
is enabled.  (2) Because they are now read-only, the ".imposter" command
in the CLI no longer requires the --unsafe-testing command-line option.
(3) Imposter tables do not participate in a PRAGMA integrity_check and hence
do not cause false-positive errors.  However, you can still do an
integrity_check on the the imposter table itself by naming the imposter
table as the argument to integrity_check.

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