store/postgres: warn if locale is not en_US.utf8

[saihaj]

Check for user's locale on startup and if it is not en_US.utf8 panic.

Closes #3118

[leoyvens]

We need a guide for what an indexer should do when encountering this error, and the error message should link to that guide.

[azf20]

@saihaj from discussion with @leoyvens , I think we may want to make the error message a bit more informative (i.e. directing the user to change their DB locale to match, or change the env variable)

[lutter]

Here's how I think we should try and manage using the same locale for all indexers. We'll probably be stuck on en_US.UTF-8 because there are fewer indexers that need to change, though C would be more preferable for performance.

1. Communicate a timeline and some writeup on what indexers need to do
2. Print a warning when the locale is not the specified one for a while (probably months, definitely two releases)
3. Switch to enforce the locale, i.e., the current behavior of panicking on startup

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[saihaj]

#4163