refactor: use b.Loop() to simplify the code and improve performance

[promalert]

Before asking for reviews, here is a check list of the most common things you might need to consider:

• updating the Changelog
• writing unit tests
• checking if your changes affect other coins or tokens in unintended ways
• testing on multiple environments (Qt, Android, ...)
• having an AI review your changes

These changes use b.Loop() to simplify the code and improve performance
Supported by Go Team, more info: https://go.dev/blog/testing-b-loop 

Before:

go test -run=^$ -bench=. ./backend/rates     
goos: darwin
goarch: arm64
pkg: github.com/BitBoxSwiss/bitbox-wallet-app/backend/rates
cpu: Apple M4
BenchmarkBoltDump/individual-10  	     108	  10673439 ns/op
BenchmarkBoltDump/json-10        	     102	  12105460 ns/op
BenchmarkBoltDump/gob-10         	     115	   9884914 ns/op
BenchmarkBoltLoad/individual-10  	   15694	     73471 ns/op
BenchmarkBoltLoad/json-10        	     748	   1925352 ns/op
BenchmarkBoltLoad/gob-10         	    5659	    202964 ns/op
Logging into '/Users/mac/Library/Application Support/bitbox/log.txt' from 'debug'.
BenchmarkDumpHistoryBucket-10    	      92	  11816275 ns/op
BenchmarkLoadHistoryBucket-10    	   15392	     76318 ns/op
PASS
ok  	github.com/BitBoxSwiss/bitbox-wallet-app/backend/rates	14.947s

After:

 go test -run=^$ -bench=. ./backend/rates             
goos: darwin
goarch: arm64
pkg: github.com/BitBoxSwiss/bitbox-wallet-app/backend/rates
cpu: Apple M4
BenchmarkBoltDump/individual-10  	     110	  11288706 ns/op
BenchmarkBoltDump/json-10        	     106	  11686261 ns/op
BenchmarkBoltDump/gob-10         	     116	  10437647 ns/op
BenchmarkBoltLoad/individual-10  	   16374	     73399 ns/op
BenchmarkBoltLoad/json-10        	     762	   1553812 ns/op
BenchmarkBoltLoad/gob-10         	    5810	    204027 ns/op
Logging into '/Users/mac/Library/Application Support/bitbox/log.txt' from 'debug'.
BenchmarkDumpHistoryBucket-10    	     110	  10907391 ns/op
BenchmarkLoadHistoryBucket-10    	   15259	     76367 ns/op
PASS
ok  	github.com/BitBoxSwiss/bitbox-wallet-app/backend/rates	13.338s

[promalert]

@benma Thanks! If there's anything need I to do, please feel free to let me know.