Try next host after a "cannot connect now" failure.

If a server returns ERRCODE_CANNOT_CONNECT_NOW, try the next host,
if multiple host names have been provided.  This allows dealing
gracefully with standby servers that might not be in hot standby mode
yet.

In the wake of the preceding commit, it might be plausible to retry
many more error cases than we do now, but I (tgl) am hesitant to
move too aggressively on that --- it's not clear it'd be desirable
for cases such as bad-password, for example.  But this case seems
safe enough.

Hubert Zhang, reviewed by Takayuki Tsunakawa

Discussion: https://postgr.es/m/BN6PR05MB3492948E4FD76C156E747E8BC9160@BN6PR05MB3492.namprd05.prod.outlook.com

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