Replace InvoicePayer with utility methods

[valentinewallace]

We're moving payment retries to ChannelManager in preparation for supporting trampoline payments.

Partially addresses #1932

Based on #2002 with another small commit taken from #1996

[codecov-commenter]

Codecov Report

Base: 90.89% // Head: 91.07% // Increases project coverage by +0.17% 🎉

Coverage data is based on head (b7c5e47) compared to base (760ab65).
Patch has no changes to coverable lines.

❗ Current head b7c5e47 differs from pull request most recent head a23b5eb. Consider uploading reports for the commit a23b5eb to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2007      +/-   ##
==========================================
+ Coverage   90.89%   91.07%   +0.17%     
==========================================
  Files          99       99              
  Lines       53026    51731    -1295     
  Branches    53026    51731    -1295     
==========================================
- Hits        48197    47113    -1084     
+ Misses       4829     4618     -211     

Impacted Files	Coverage Δ
lightning-background-processor/src/lib.rs	86.02% <ø> (-0.33%)	⬇️
lightning-invoice/src/payment.rs	90.44% <ø> (+0.78%)	⬆️
lightning-invoice/src/utils.rs	97.59% <ø> (-0.05%)	⬇️
lightning/src/util/fairrwlock.rs	70.58% <0.00%> (-17.65%)	⬇️
lightning/src/util/events.rs	29.56% <0.00%> (-1.64%)	⬇️
lightning/src/ln/outbound_payment.rs	89.38% <0.00%> (-0.80%)	⬇️
lightning-invoice/src/lib.rs	86.32% <0.00%> (-0.49%)	⬇️
lightning/src/chain/channelmonitor.rs	91.02% <0.00%> (-0.10%)	⬇️
lightning/src/ln/functional_tests.rs	96.97% <0.00%> (-0.10%)	⬇️
lightning/src/routing/scoring.rs	97.31% <0.00%> (-0.02%)	⬇️
... and 5 more

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[valentinewallace]

Rebased so I could get the changes from #1873 in #2008

[valentinewallace]

Fixed bad doc link

diff --git a/lightning-invoice/src/payment.rs b/lightning-invoice/src/payment.rs
index 787219d88..6b70d960b 100644
--- a/lightning-invoice/src/payment.rs
+++ b/lightning-invoice/src/payment.rs
@@ -257,12 +257,16 @@ pub enum PaymentError {
 /// Useful for unit testing internal methods.
 trait Payer {
        /// Sends a payment over the Lightning Network using the given [`Route`].
+       ///
+       /// [`Route`]: lightning::routing::router::Route
        fn send_payment(
                &self, payment_hash: PaymentHash, payment_secret: &Option<PaymentSecret>,
                payment_id: PaymentId, route_params: RouteParameters, retry_strategy: Retry
        ) -> Result<(), PaymentError>;

        /// Sends a spontaneous payment over the Lightning Network using the given [`Route`].
+       ///
+       /// [`Route`]: lightning::routing::router::Route
        fn send_spontaneous_payment(
                &self, payment_preimage: PaymentPreimage, payment_id: PaymentId, route_params: RouteParameters,
                retry_strategy: Retry

[dunxen]

Looks good from a first review

[TheBlueMatt]

LGTM feel free to squash. I think we need to migrate considers_inflight_htlcs_between_retries and fails_retrying_invoice_after_expiration but that can come in a followup.