CLI API refactoring and improvement (#4692)

It changes the way we extended the CLI functionalities of substrate to allow more flexibility. (If this was not clear, here is another version: it changes the `sc_cli` API to allow more flexibility).

This touches a few important things:
 - the startup of the async task with tokei:
    This was in node and node-template and I moved it to substrate. The idea is to have 1 time the code that handles unix signals (SIGTERM and SIGINT) properly. It is however possible to make this more generic to wait for a future instead and provide only a helper for the basic handling of SIGTERM and SIGINT.
 - increased the version of structopt and tokei
 - no more use of structopt internal's API
 - less use of generics

Related to #4643 and https://github.com/paritytech/cumulus/pull/42: the implementation of "into_configuration" and "get_config" are similar but with better flexibility so it is now possible in cumulus to have the command-line arguments only of the run command for polkadot if we want

Related to https://github.com/paritytech/cumulus/issues/24 and https://github.com/paritytech/cumulus/issues/34 : it will now be possible to make a configuration struct for polkadot with some overrides of the default parameters much more easily.

substrate/client/cli/Cargo.toml

@@ -17,7 +17,7 @@ time = "0.1.42"
 ansi_term = "0.12.1"
 lazy_static = "1.4.0"
 app_dirs = "1.2.1"
-tokio = "0.2"
+tokio = { version = "0.2.9", features = [ "signal", "rt-core", "rt-threaded" ] }
 futures = "0.3.1"
 fdlimit = "0.1.1"
 serde_json = "1.0.41"
@@ -32,8 +32,9 @@ sp-state-machine = { version = "0.8", path = "../../primitives/state-machine" }
 sc-telemetry = { version = "2.0.0", path = "../telemetry" }
 sp-keyring = { version = "2.0.0", path = "../../primitives/keyring" }
 names = "0.11.0"
-structopt = "=0.3.7"
+structopt = "0.3.8"
 sc-tracing = { version = "2.0.0", path = "../tracing" }
+chrono = "0.4.10"
 
 [target.'cfg(not(target_os = "unknown"))'.dependencies]
 rpassword = "4.0.1"