Create TTL map with time wheel architecture

This change adds a DashMap-like struct which has a background tasks to clean
up entries that have outlived a configurable TTL.

This struct is simliar to https://github.com/moka-rs/moka, which also uses
time wheels. Having our own module avoids introducing a large dependency, which
keeps this project closer to vanilla datafusion.

This change is meant to be useful for #89, where it's
possible for `ExecutionStages` to be orphaned in `ArrowFlightEndpoint`. We need an async task to clean up old entries.

Informs: #90

Author: jayshrivastava

src/common/mod.rs

@@ -1 +1,2 @@
+pub mod ttl_map;
 pub mod util;