Add a safety guard against dupe sk in walproposer config

[petuhovskiy]

Steps to reproduce

Start a walproposer with an invalid config (duplicate safekeeper) in postgresql.conf, like:

neon.safekeepers = 'safekeeper-1.us-east-2.aws.neon.build:6401,safekeeper-3.us-east-2.aws.neon.build:6401,safekeeper-1.us-east-2.aws.neon.build:6401'

This is not a new problem, but I haven't found an existing issue for it.

Expected result

walproposer exits with FATAL error because it detects a duplicate safekeeper in the configuration, data on safekeepers is not modified.

Implementation can compare hostname or nodeId received in each connection (latter is more safe).

Actual result

Probably everything explodes and corrupts data