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/bin/node-template/Cargo.toml

@@ -12,9 +12,7 @@ path = "src/main.rs"
 
 [dependencies]
 futures = "0.3.1"
-ctrlc = { version = "3.1.3", features = ["termination"] }
 log = "0.4.8"
-tokio = { version = "0.2", features = ["rt-threaded"] }
 sc-cli = { version = "0.8.0", path = "../../client/cli" }
 sp-core = { version = "2.0.0", path = "../../primitives/core" }
 sc-executor = { version = "0.8", path = "../../client/executor" }
@@ -31,7 +29,8 @@ grandpa-primitives = { version = "2.0.0", package = "sp-finality-grandpa", path
 sc-client = { version = "0.8", path = "../../client/" }
 node-template-runtime = { version = "2.0.0", path = "runtime" }
 sp-runtime = { version = "2.0.0", path = "../../primitives/runtime" }
-sc-basic-authorship = { path  = "../../client/basic-authorship" }
+sc-basic-authorship = { path = "../../client/basic-authorship" }
+structopt = "0.3.8"
 
 [build-dependencies]
 vergen = "3.0.4"