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/cli/Cargo.toml

@@ -31,7 +31,7 @@ hex-literal = "0.2.1"
 jsonrpc-core = "14.0.3"
 log = "0.4.8"
 rand = "0.7.2"
-structopt = "=0.3.7"
+structopt = { version = "0.3.8", optional = true }
 
 # primitives
 sp-authority-discovery = { version = "2.0.0",  path = "../../../primitives/authority-discovery" }
@@ -81,9 +81,7 @@ node-primitives = { version = "2.0.0", path = "../primitives" }
 node-executor = { version = "2.0.0", path = "../executor" }
 
 # CLI-specific dependencies
-tokio = { version = "0.2", features = ["rt-threaded"], optional = true }
 sc-cli = { version = "0.8.0", optional = true, path = "../../../client/cli" }
-ctrlc = { version = "3.1.3", features = ["termination"], optional = true }
 node-transaction-factory = { version = "0.8.0", optional = true, path = "../transaction-factory" }
 
 # WASM-specific dependencies
@@ -99,10 +97,19 @@ futures = "0.3.1"
 tempfile = "3.1.0"
 
 [build-dependencies]
-sc-cli = { version = "0.8.0", package = "sc-cli", path = "../../../client/cli" }
 build-script-utils = { version = "2.0.0", package = "substrate-build-script-utils", path = "../../../utils/build-script-utils" }
-structopt = "=0.3.7"
-vergen = "3.0.4"
+structopt = { version = "0.3.8", optional = true }
+node-transaction-factory = { version = "0.8.0", optional = true, path = "../transaction-factory" }
+
+[build-dependencies.sc-cli]
+version = "0.8.0"
+package = "sc-cli"
+path = "../../../client/cli"
+optional = true
+
+[build-dependencies.vergen]
+version = "3.0.4"
+optional = true
 
 [features]
 default = ["cli", "wasmtime"]
@@ -114,10 +121,10 @@ browser = [
 cli = [
 	"sc-cli",
 	"node-transaction-factory",
-	"tokio",
-	"ctrlc",
 	"sc-service/rocksdb",
 	"node-executor/wasmi-errno",
+	"vergen",
+	"structopt",
 ]
 wasmtime = [
 	"cli",