fix concurrency issue in SumComputeTime

[zkokelj]

Why this change is needed

Last week I gateway crashed with the following error:

fatal error: concurrent map iteration and map write

goroutine 684 [running]:
github.com/ten-protocol/go-ten/tools/walletextension/ratelimiter.(*RateLimiter).SumComputeTime(0xc000440180, {0xb7, 0xe8, 0x4, 0xf1, 0x51, 0x44, 0xee, 0xe7, 0x5, ...})
	/home/obscuro/go-obscuro/tools/walletextension/ratelimiter/rate_limiter.go:108 +0x10f
github.com/ten-protocol/go-ten/tools/walletextension/ratelimiter.(*RateLimiter).Allow(0xc000440180, {0xb7, 0xe8, 0x4, 0xf1, 0x51, 0x44, 0xee, 0xe7, 0x5, ...})
	/home/obscuro/go-obscuro/tools/walletextension/ratelimiter/rate_limiter.go:197 +0x1a8
github.com/ten-protocol/go-ten/tools/walletextension/rpcapi.ExecAuthRPC[...]({0x1762fd0, 0xc0037ee690}, 0xc0005c0000, 0xc000297400, {0x13345f3, 0xe}, {0xc003a5cde0, 0x2, 0x2})
	/home/obscuro/go-obscuro/tools/walletextension/rpcapi/utils.go:107 +0x1a8

I found the issue and fixed it in this PR.

What changes were made as part of this PR

Please provide a high level list of the changes made

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[tudor-malene]

lgtm