Move the background jobs to another crate

I wrote this code intending to extract it, and I'm planning to release
it after a few more changes which aren't relevant to crates.io. The code
right now is largely the same as what was here before, the main
difference being the use of a different error type.

The error handling will probably churn some more in the near future as I
figure out what I want the error handling story to look like in that
crate

Author: sgrif

Cargo.lock

@@ -49,6 +49,7 @@ dotenv = "0.11.0"
 toml = "0.4"
 diesel = { version = "1.4.0", features = ["postgres", "serde_json", "chrono", "r2d2"] }
 diesel_full_text_search = "1.0.0"
+swirl = { git = "https://github.com/sgrif/swirl.git" }
 serde_json = "1.0.0"
 serde_derive = "1.0.0"
 serde = "1.0.0"
@@ -63,7 +64,6 @@ tempdir = "0.3.7"
 parking_lot = "0.7.1"
 jemallocator = { version = "0.1.8", features = ['unprefixed_malloc_on_supported_platforms', 'profiling'] }
 jemalloc-ctl = "0.2.0"
-threadpool = "1.7"
 
 lettre = {git = "https://github.com/lettre/lettre", version = "0.9"}
 lettre_email = {git = "https://github.com/lettre/lettre", version = "0.9"}