Make sure to avoid double refresh in background

[DoumanAsh]

I was reviewing code and found potential race when performing background refresh.

So I modified test test_background_refresh_and_expire to try to simulate race between multiple tasks attempting to perform background refresh in parallel

Even under release build it is difficult to have exact reproduction

19:51 $ cargo test --release
warning: method `is_refreshing` is never used
  --> src/cache/mod.rs:96:14
   |
95 | impl JWKSCache {
   | -------------- method in this implementation
96 |     const fn is_refreshing(&self) -> bool {
   |              ^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: `jsonwebtoken-jwks-cache` (lib) generated 1 warning
warning: `jsonwebtoken-jwks-cache` (lib test) generated 1 warning (1 duplicate)
    Finished `release` profile [optimized] target(s) in 0.09s
     Running unittests src/lib.rs (target/release/deps/jsonwebtoken_jwks_cache-bb5944cb4e7a0136)

running 6 tests
test pem_set::test::test_pem_map ... ok
test cache::test::test_fetch_concurrent_from_empty ... ok
test cache::test::test_background_refresh_and_expire ... FAILED
test cache::test::test_reqwest_gcp_jwk_integration ... ok
test cache::test::test_timeout_policy ... ok
test cache::test::test_reqwest_gcp_pub_keys_integration ... ok

failures:

---- cache::test::test_background_refresh_and_expire stdout ----

thread 'cache::test::test_background_refresh_and_expire' (63294) panicked at src/cache/test.rs:161:5:
assertion `left == right` failed: Should have refreshed from IO once
  left: 4
 right: 3
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I also added multiple parallel runs to test_timeout_policy to make sure it works ok under race friendly condition

[DoumanAsh]

I also made safeguard to ensure refershing will not stuck and continue its attempts until JWKs get expired
dd0239c

[DoumanAsh]

One more commit to introduce state guard when immediate cache update is performed to make sure state is reset even if future gets cancelled