perf: Make the UUIDv7 implementation lock-free

[heyihong]

This lock-free implementation leverages atomic operations to avoid locking and context switching. Besides, it uses the sequence number exclusively, rather than relying on nanosecond-level information. Based on microbenchmarks, it can generate more UUIDv7 values within a given period of time.

Before:

go test -bench=BenchmarkUUID_NewV7 -benchtime=10s -run=NONE

goos: darwin
goarch: arm64
pkg: github.com/google/uuid
BenchmarkUUID_NewV7-8         	17931745	       664.1 ns/op
BenchmarkUUID_NewV7Pooled-8   	66918664	       180.4 ns/op

After:

go test -bench=BenchmarkUUID_NewV7 -benchtime=10s -run=NONE

goos: darwin
goarch: arm64
pkg: github.com/google/uuid
BenchmarkUUID_NewV7-8         	18219297	       655.6 ns/op
BenchmarkUUID_NewV7Pooled-8   	71690725	       167.7 ns/op

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