cmd,eth: 16400 Add an option to stop geth once in sync. WIP for light mode (#17321)

* cmd, eth: Added in the flag to step geth once sync based on input

* cmd, eth: 16400 Add an option to stop geth once in sync.

* cmd: 16400 Add an option to stop geth once in sync. WIP

* cmd/geth/main, les/fletcher: added in light mode support

* cmd/geth/main, les/fletcher: Cleaned Comments and code for light mode

* cmd: 16400 Fixed formatting issue and cleaned code

* cmd, eth, les: 16400 Fixed formatting issues

* cmd, eth, les: Performed gofmt to update formatting

* cmd, eth, les: Fixed bugs resulting formatting

* cmd/geth, eth/, les: switched to downloader event

* eth: Fixed styling and gen_config

* eth/: Fix nil error in config file

* cmd/geth: Updated countdown log

* les/fetcher.go: Removed depcreated channel

* eth/downloader.go: Removed deprecated select

* cmd/geth, cmd/utils: Fixed minor issues

* eth: Reverted config files to proper format

* eth: Fixed typo in config file

* cmd/geth, eth/down: Updated code to use header time stamp

* eth/downloader: Changed the time threshold to 10 minutes

* cmd/geth, eth/downloader: Updated downloading event to pass latest header

* cmd/geth: Updated main to use right timer object

* cmd/geth: Removed unused failed event

* cmd/geth: added in correct time field with type assertion

* cmd/geth, cmd/utils: Updated flag to use boolean

* cmd/geth, cmd/utils, eth/downloader: Cleaned up code based on recommendations

* cmd/geth: Removed unneeded import

* cmd/geth, eth/downloader: fixed event field and suggested changes

* cmd/geth, cmd/utils: Updated flag and linting issue

eth/downloader/events.go

@@ -16,6 +16,10 @@
 
 package downloader
 
-type DoneEvent struct{}
+import "github.com/ethereum/go-ethereum/core/types"
+
+type DoneEvent struct {
+	Latest *types.Header
+}
 type StartEvent struct{}
 type FailedEvent struct{ Err error }