Make futures-channel dep optional and required for mysql feature

Author: nickelc

Cargo.toml

@@ -15,7 +15,7 @@ description = "An async extension for Diesel the safe, extensible ORM and Query
 [dependencies]
 diesel = { version = "2.0.0", default-features = false, features = ["i-implement-a-third-party-backend-and-opt-into-breaking-changes"]}
 async-trait = "0.1.51"
-futures-channel = { version = "0.3.17", default-features = false, features = ["std", "sink"] }
+futures-channel = { version = "0.3.17", default-features = false, features = ["std", "sink"], optional = true }
 futures-util = { version = "0.3.17", default-features = false, features = ["std", "sink"] }
 tokio-postgres = { version = "0.7.2", optional = true}
 tokio = { version = "1", optional = true}
@@ -35,7 +35,7 @@ diesel = { version = "2.0.0", default-features = false,  features = ["chrono"]}
 
 [features]
 default = []
-mysql = ["diesel/mysql_backend", "mysql_async", "mysql_common"]
+mysql = ["diesel/mysql_backend", "mysql_async", "mysql_common", "futures-channel"]
 postgres = ["diesel/postgres_backend", "tokio-postgres", "tokio", "tokio/rt"]
 
 [[test]]