Add retry engine

[arlyon]

This PR adds the ability to have the client automatically apply retries by applying a RequestStrategy. Closes #178

• refactor common client code into single struct
• add idempotency strategy
• add retries strategy
• port async-std client
• port tokio and tokio-blocking clients
• restore set_stripe_account api
• determine time-out function
• fill out documentation
• add examples
• clone requests correctly

There are 4 strategies:

Once

This is the basic usage (and the default). This strategy will attempt to make a request, and immediately return control to the caller when it fails.

Idempotent

This is one step up from Once. Using this strategy will use a idempotency key of your choice, and make requests with it. Failures will continue to return control to the caller, however you may retry the request at will.

Retry (not recommended)

This strategy will make the same request up to n times, immediately one-after-the-other. This strategy is not recommended for production use as it is bad manners. Regardless, it may be useful for testing.

Exponential Back-off

This strategy inherits all the semantics of the Retry strategy, but with an increasing wait between requests.

🚨 Note: in both the Retry and ExponentialBackoff strategies n is the maximum limit. If the stripe API explicitly communicates to the client that a retry is not useful (via the Stripe-Should-Retry header), the request will be aborted without reaching the limit.

Usage

// create a client, and use the ExponentialBackoff strategy for all subsequent requests
let client = Client::new("sk_1234").with_strategy(RequestStrategy::ExponentialBackoff(5));

// we need manual control, let's make a new client and use the idempotent strategy
let idempotent = client.clone().with_strategy(RequestStrategy::Idempotent("my_token"));

[arlyon]

@FL33TW00D ^

[FL33TW00D]

Looks really clean, few things failing but apart from that.

[FL33TW00D]

Removed this intentionally?

[arlyon]

Missing some final issues around cloning requests. Basically cloning an http-types::Request truncates the body, so you need to re-add it. Not a tough fix but it'll have to wait for the moment!

[codecov]

Codecov Report

Merging #184 (cc05fda) into master (650280b) will increase coverage by 1.70%.
The diff coverage is 85.75%.

@@            Coverage Diff            @@
##           master    #184      +/-   ##
=========================================
+ Coverage    4.34%   6.04%   +1.70%     
=========================================
  Files         122     124       +2     
  Lines       10551   10624      +73     
=========================================
+ Hits          458     642     +184     
+ Misses      10093    9982     -111     

Impacted Files	Coverage Δ
src/error.rs	0.00% <0.00%> (ø)
src/resources/charge_ext.rs	0.00% <ø> (ø)
src/resources/customer_ext.rs	0.00% <ø> (ø)
src/resources/generated/account.rs	1.86% <ø> (ø)
src/resources/generated/account_link.rs	0.00% <ø> (ø)
src/resources/generated/application_fee.rs	0.00% <ø> (ø)
src/resources/generated/balance.rs	0.00% <ø> (ø)
src/resources/generated/balance_transaction.rs	0.00% <ø> (ø)
src/resources/generated/billing_portal_session.rs	0.00% <ø> (ø)
src/resources/generated/charge.rs	0.67% <ø> (ø)
... and 59 more

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