Conn-check

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- Added --max-timeout CLI option to restrict maximum execution time.
- Added connection timeouts to HTTP and PostgreSQL checks.
- Added --connect-timeout CLI option to set global connection timeout.
- Added timeout option to each individual check to override global connection timeout.

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- Switched to using txrequests for HTTP requests with better proxy support.
- Fixed UDP checks targetting host rather than IP if available.
- Fixed initial TCP check for HTTP checks targetting upstream instead of proxy.

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- Fixed HTTP proxy error in HTTP checks due to typo.

- Fixed dependencies when installing from local dir.
- Made improvements to readme.

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- Added optional headers and body arguments to HTTP checks.

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- Added HTTP proxy support to http checks
- Fixed issue with loading duplicate SSL CA certificates, and added flag to load from a custom dir