312 use stellar fee statistics to derive transaction fee

[b-yap]

closes #312

General overview of the changes:

1. Remove DEFAULT_STROOP_FEE_PER_OPERATION constant in production code.
2. Remove stroop_fee_per_operation parameters on method fn send_to_address(); since
3. fee will be queried from horizon; the method is calledfn get_fee_stats()
4. Memoization using the cached lib to return the same result until 10 minutes (or 600 seconds) have elapsed.

todo: only memoize for Ok() results.

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How to begin the review:

1. clients/wallet/src/types.rs -> has the FeeAttribute enum, where you can pick which one to base the operation on
2. clients/wallet/src/horizon/responses.rs -> Defining the response into a struct of FeeStats and FeeDistribution
3. clients/wallet/src/horizon/serde.rs -> Implements deserialization of the fields in FeeStats and FeeDistribution to u32
4. clients/wallet/src/horizon/traits.rs -> add another method to the trait: fn get_fee_stats() and should return the struct FeeStats
5. clients/wallet/src/horizon/horizon.rs -> implements fn get_fee_stats()
6. clients/wallet/src/horizon/tests.rs -> testing the method fn get_fee_stats()
7. clients/wallet/src/stellar_wallet.rs
   • the method fn get_fee() is introduced, with caching:

        #[cached(result = true, time = 600)]
     

   • the method fn get_fee() is called inside fn send_to_address(...), removing the stroop_fee_per_operation param
8. ALL calls of fn send_to_address(...) will not need to pass a stroop_fee_per_operation

[ebma]

I love it, these are some really great changes 🎉 Left some minor comments

[ebma]

LGTM ready to merge once CI passed