Fix concurrent SetTags race on config.Tags

[BetterAndBetterII]

Description

Serf.SetTags wrote config.Tags without synchronization even though the Serf godoc states that all methods are safe to call concurrently. Concurrent SetTags calls (and concurrent reads via NodeMeta / query tag filters) race on the tag map, and concurrent memberlist.UpdateNode calls can also race.

This change:

• Protects config.Tags with tagsLock
• Snapshots tags under the lock for NodeMeta and query tag filters
• Serializes memberlist.UpdateNode from SetTags

Related Issue

Fixes #621

How Has This Been Tested?

go test -race -count=1 -timeout 90s -run 'TestSerf_SetTags$|TestSerf_SetTags_Concurrent' ./serf
go test -race -count=1 -timeout 120s ./serf

Added TestSerf_SetTags_Concurrent, which failed under -race before the lock and passes after.

Contributor Checklist

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