services: use wants dependency where possible

Let A be a service that depends on another service B.
When A can gracefully handle failures and restarts of B, use
```
wants = [ "B.service" ];
after = [ "B.service" ];
```
instead of
```
requires = [ "B.service" ];
after = [ "B.service" ];
```
in the definition of A.

This way, A keeps running when B is stopped or restarted after a failure.
With `requires`, A is instead stopped when B is stopped or restarted due to a failure.

This brings two benefits:

1. Improved uptime
Examples:
- RTL keeps running when one lightning node has failed
- btcpayserver keeps running and accepting on-chain payments when the lightning node has crashed

2. Avoids a systemd bug where depending units (`A.service` in the
above example) are not restarted when their dependency fails
(issue github/systemd#18856, no full link to avoid spamming the issue).
In real world nix-bitcoin deployments, this issue was only likely to
appear when clightning failed during activation, causing depending
units (like `RTL`) to stop and not be restarted.
All services depending on `clightning` have now been changed to use
`wants`, thereby avoiding the bug.

Services `electrs` and `lightning-loop` fail when their respective
dependencies stop, so these services have not been changed.
I also haven't changed services `joinmarket` and
`joinmarket-yieldgenerator`. Further manual testing is needed to
determine if they can be switched to `wants`.

modules/mempool.nix

@@ -275,10 +275,11 @@ in {
       };
     };
 
-    systemd.services.mempool = {
+    systemd.services.mempool = rec {
       wantedBy = [ "multi-user.target" ];
-      requires = [ "${cfg.electrumServer}.service" ];
-      after = [ "${cfg.electrumServer}.service" "mysql.service" ];
+      requires = [ "mysql.service" ];
+      wants = [ "${cfg.electrumServer}.service" ];
+      after = requires ++ wants;
       preStart = ''
         mkdir -p '${cacheDir}/cache'
         <${configFile} sed \