client/cli/src/config: Warn on low file descriptor limit (#6956)

* client/cli/src/config: Warn on low file descriptor limit

Substrate sets the soft file descriptor limit to the hard limit at
startup. In the case of the latter being low already (< 10_000) a
Substrate node under high demand might run into issues e.g. when opening
up new TCP connections or persisting data to the database.

With this commit a warn message is printed to stderr.

* client/cli/Cargo.toml: Update to fdlimit 0.2.0

substrate/client/service/test/Cargo.toml

@@ -18,7 +18,7 @@ tokio = "0.1.22"
 futures01 = { package = "futures", version = "0.1.29" }
 log = "0.4.8"
 env_logger = "0.7.0"
-fdlimit = "0.1.4"
+fdlimit = "0.2.0"
 parking_lot = "0.10.0"
 sc-light = { version = "2.0.0-rc6", path = "../../light" }
 sp-blockchain = { version = "2.0.0-rc6", path = "../../../primitives/blockchain" }