proxy: lazily parse startup pg params

[conradludgate]

Problem

proxy params being a HashMap<String,String> when it contains just

application_name: psql
database: neondb
user: neondb_owner

is quite wasteful allocation wise.

Summary of changes

Keep the params in the wire protocol form, eg:

application_name\0psql\0database\0neondb\0user\0neondb_owner\0

Using a linear search for the map is fast enough at small sizes, which is the normal case.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3150 tests run: 3017 passed, 0 failed, 133 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_statvfs_pressure_usage: debug

Code coverage* (full report)

• functions: 31.4% (6485 of 20666 functions)
• lines: 48.4% (50207 of 103758 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9490d93 at 2024-05-30T11:10:52.147Z :recycle:}