Ensure that all ast_datastore_info structures are 'const'.

Ensure that all ast_datastore_info structures are 'const'.

While addressing a bug, I came across a instance of 'struct ast_datastore_info'
that was not declared 'const'. Since the API already expects them to be
'const', this patch changes the declarations of all existing instances
that were not already declared that way.
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Merged revisions 370183 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 370184 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370187 65c4cc65-6c06-0410-ace0-fbb531ad65f3