lnwallet: add configurable cache for web fee estimator

Add fee.min-update-timeout and fee.max-update-timeout config options to
allow configuration of the web fee estimator cache.

sample-lnd.conf

@@ -305,14 +305,28 @@
 ; The default value below is 20 MB (1024 * 1024 * 20)
 ; blockcachesize=20971520
 
-; Optional URL for external fee estimation. If no URL is specified, the method
-; for fee estimation will depend on the chosen backend and network. Must be set
-; for neutrino on mainnet.
+; DEPRECATED: Use 'fee.url' option. Optional URL for external fee estimation.
+; If no URL is specified, the method for fee estimation will depend on the
+; chosen backend and network. Must be set for neutrino on mainnet.
 ; Default:
 ;   feeurl=
 ; Example:
 ;   feeurl=https://nodes.lightning.computer/fees/v1/btc-fee-estimates.json
 
+; Optional URL for external fee estimation. If no URL is specified, the method
+; for fee estimation will depend on the chosen backend and network. Must be set
+; for neutrino on mainnet.
+; Default:
+;   fee.url=
+; Example:
+;   fee.url=https://nodes.lightning.computer/fees/v1/btc-fee-estimates.json
+
+; The minimum interval in which fees will be updated from the specified fee URL.
+; fee.min-update-timeout=5m
+
+; The maximum interval in which fees will be updated from the specified fee URL.
+; fee.max-update-timeout=20m
+
 ; If true, then automatic network bootstrapping will not be attempted. This
 ; means that your node won't attempt to automatically seek out peers on the
 ; network.