Fix bug #184

Add specialized ExecuteBlindSuppressTimeout(), use for Rollback() to prevent error when backend is in aborted state.
Connection Timeout

Remove support for custom query timeouts for internal queries; internal queries always get 20 seconds.
Fix bogus -1 timeout.
Revert warning 1591 suppression.
Merge pull request #185 from npgsql/fix_bug_184

Fix bug #184