Use new Replica::pending_tasks in TDB2::pending_tasks

[djmitche]

In GothenburgBitFactory/taskchampion#452, @Scandiravian created new Replica methods to return just the pending tasks, in an efficient single query.

Let's use that to implement TDB2::pending_tasks.

This will require a new release of TaskChampion before it lands. It's possible to start work on developing the patch before that something like

diff --git src/taskchampion-cpp/Cargo.toml src/taskchampion-cpp/Cargo.toml
index 91e557587..a7e0ad1ef 100644
--- src/taskchampion-cpp/Cargo.toml
+++ src/taskchampion-cpp/Cargo.toml
@@ -3,13 +3,13 @@ name = "taskchampion-lib"
 version = "0.1.0"
 edition = "2021"
 publish = false
 
 [lib]
 crate-type = ["staticlib"]
 
 [dependencies]
-taskchampion = "0.7.0"
+taskchampion = { path = "../../../taskchampion/taskchampion/" }
 cxx = "1.0.124"
 
 [build-dependencies]
 cxx-build = "1.0"

where the path points to https://github.com/GothenburgBitFactory/taskchampion/tree/main/taskchampion