Close heartbeat connection on write error

[cristinaleonr]

This PR adds logging and closes the heartbeat connection whenever there is an error writing to Memorystore.

It also adds a conn interface for the heartbeat handler to take in.

Related to #114.

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 877

• 18 of 20 (90.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.6%) to 97.924%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
handler/heartbeat.go	15	17	88.24%

Totals
Change from base Build 871:	1.6%
Covered Lines:	1604
Relevant Lines:	1638

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💛 - Coveralls

[stephen-soltesz]

Suggested to avoid double logging.

Reviewed 2 of 3 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @cristinaleonr)

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heartbeat/heartbeat.go line 71 at r1 (raw file):

	hostname := rm.Hostname
	if err := h.Put(hostname, "Registration", &rm, true); err != nil {
		log.Printf("failed to write Registration message to Memorystore, err: %v", err)

These will double log the err here and in closeConnection. I recommend wrapping the err in a new error and returning that so there is one log event.

e.g. err = fmt.Errorf("%w: failed to write ... ", err)

[cristinaleonr]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @stephen-soltesz)

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heartbeat/heartbeat.go line 71 at r1 (raw file):

Previously, stephen-soltesz (Stephen Soltesz) wrote…

These will double log the err here and in closeConnection. I recommend wrapping the err in a new error and returning that so there is one log event.

e.g. err = fmt.Errorf("%w: failed to write ... ", err)

Done.