Rename ConfigHeartbeatInterval to HeartbeatConfigInterval

"Config" and "Lock" are reserved keywords in the .NET configuration API
and variable names cannot start woth either of them. It's a run time
issue and this fix resolves it by renaming the variable names.

Change-Id: Iea00f8ac05e2d04c3e55726fc272ff5f9bc0a556
Reviewed-on: http://review.couchbase.org/41828
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>