[repoapi] add an anonymous_cnx function
This function return an anonymous ClientConnection. It aims to replace
``dbapi.anonymous_session``.
\.svn
^build$
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\.pyc$
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\~$
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\.swp$
^doc/book/en/apidoc$
\.old$
syntax: regexp
.*/data/database/.*\.sqlite
.*/data/database/.*\.config
.*/data/database/tmpdb.*
.*/data/ldapdb/.*
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/