rdbms: Clean up MssqlBlob constructor

rdbms: Clean up MssqlBlob constructor

Returning inside PHP constructors doesn't work, so whatever this code
was previously trying to do didn't work. Now we directly set $this->data
to the existing $data->data. Note that we can't fall through to the
instanceof Blob due to MssqlBlob->fetch() not being idempotent.

Also expand the @param documentation for $data.

h/t to phan for highlighting this and Anomie for explaining that the
behavior was wrong and how to properly fix it.

Change-Id: Ied5ba2f63cbaf7c8e0f951000dbc4aa3089edd80