feat: track server last sent/received

[levkk]

Description

• Track last_sent / last_received for debugging stuck connections
• Remove contention from server stats mutex - one mutex per connection for stats instead
• time stats are now subms precision (e.g. 32.342ms) including in admin DB and Prometheus

[codecov]

Codecov Report

❌ Patch coverage is 77.18631% with 60 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/admin/show_servers.rs	0.00%	29 Missing ⚠️
pgdog/src/stats/pools.rs	0.00%	8 Missing ⚠️
pgdog/src/admin/show_stats.rs	0.00%	5 Missing ⚠️
pgdog/src/backend/pool/guard.rs	44.44%	5 Missing ⚠️
pgdog/src/admin/show_server_memory.rs	0.00%	3 Missing ⚠️
pgdog/src/backend/pool/connection/binding.rs	50.00%	3 Missing ⚠️
pgdog/src/util.rs	0.00%	3 Missing ⚠️
pgdog/src/backend/server.rs	94.44%	2 Missing ⚠️
pgdog/src/backend/stats.rs	98.66%	2 Missing ⚠️

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