Fixed DefaultSecureMimeContext to build the cert chain outside of the private key query
Fixed DefaultSecureMimeContext to build the cert chain outside of the private key query

Fixes issue #508

As pointed out in issue #508, many SQL databases do not allow reentrant queries.

The simple fix here is to grab the data we need (certificate and prvate key)
and then break out of the query and then build the CmsSigner's certificate chain.

jstedfast/MimeKit,jstedfast/MimeKit,jstedfast/MimeKit