Bump lru to 0.7.1

The advisory script brought up a RUSTSEC error:

---
error[A001]: Use after free in lru crate
    ┌─ /home/haptop/Developer/radicle-link/Cargo.lock:207:1
    │
207 │ lru 0.6.6 registry+https://github.com/rust-lang/crates.io-index
    │ --------------------------------------------------------------- security vulnerability detected
    │
    = ID: RUSTSEC-2021-0130
    = Advisory: https://rustsec.org/advisories/RUSTSEC-2021-0130
    = Lru crate has use after free vulnerability.

      Lru crate has two functions for getting an iterator. Both iterators give
      references to key and value. Calling specific functions, like pop(), will remove
      and free the value, and but it's still possible to access the reference of value
      which is already dropped causing use after free.
    = Announcement: jeromefroe/lru-rs#120
    = Solution: Upgrade to >=0.7.1
---

This patch follows the recommended solution and pins the `lru` crate
to 0.7.1.

Cargo.toml

@@ -66,7 +66,7 @@ smol_str = { version = "0.1.21", optional = true }
 
 # Non-feature optional dependencies
 blocking = { version = "1.0.2", optional = true }
-lru = { version = "0.6.5", optional = true }
+lru = { version = "0.7.1", optional = true }
 sha2 = { version = "0.9.3", optional = true }
 futures-timer = { version = "3.0.2", optional = true }
 futures-channel = { version = "0.3.13", optional = true }