Drop mutext at txnTime

[erickpintor]

There's currently a mix of mutex and atomic semantics in the txnTime struct. With atomic references/values, there's really no need to mutex. The difference, although still pretty fast, shows up as you increase the number of processes competing for the lock:

Before:

$ go test -run '^$' -bench BenchmarkTxnTime -cpu 1,2,4,6,8,10,12,14,16,18,20,22,24
goos: linux
goarch: amd64
pkg: github.com/fauna/fauna-go
cpu: AMD Ryzen 9 5900X 12-Core Processor
BenchmarkTxnTime       	21698958	       47.81 ns/op
BenchmarkTxnTime-2     	38165878	       71.46 ns/op
BenchmarkTxnTime-4     	20483498	       65.51 ns/op
BenchmarkTxnTime-6     	18176168	       96.62 ns/op
BenchmarkTxnTime-8     	 9268669	       138.1 ns/op
BenchmarkTxnTime-10    	 7864254	       159.0 ns/op
BenchmarkTxnTime-12    	 6894296	       184.0 ns/op
BenchmarkTxnTime-14    	 5897619	       171.3 ns/op
BenchmarkTxnTime-16    	 5877307	       211.4 ns/op
BenchmarkTxnTime-18    	 6124909	       213.2 ns/op
BenchmarkTxnTime-20    	 5618040	       207.3 ns/op
BenchmarkTxnTime-22    	 6004261	       171.7 ns/op
BenchmarkTxnTime-24    	 6618184	       203.7 ns/op
PASS
ok  	github.com/fauna/fauna-go	20.667s

After:

$ go test -run '^$' -bench BenchmarkTxnTime -cpu 1,2,4,6,8,10,12,14,16,18,20,22,24
goos: linux
goarch: amd64
pkg: github.com/fauna/fauna-go
cpu: AMD Ryzen 9 5900X 12-Core Processor
BenchmarkTxnTime       	 25714206	        39.49 ns/op
BenchmarkTxnTime-2     	 44444223	        26.82 ns/op
BenchmarkTxnTime-4     	 66274556	        15.58 ns/op
BenchmarkTxnTime-6     	100000000	        10.18 ns/op
BenchmarkTxnTime-8     	151929516	        7.935 ns/op
BenchmarkTxnTime-10    	173305910	        6.856 ns/op
BenchmarkTxnTime-12    	182882767	        6.513 ns/op
BenchmarkTxnTime-14    	196377723	        6.121 ns/op
BenchmarkTxnTime-16    	210278419	        5.707 ns/op
BenchmarkTxnTime-18    	218770639	        5.491 ns/op
BenchmarkTxnTime-20    	225907432	        5.338 ns/op
BenchmarkTxnTime-22    	233047292	        5.227 ns/op
BenchmarkTxnTime-24    	236090180	        5.138 ns/op
PASS
ok  	github.com/fauna/fauna-go	21.388s