all: ensure internal subscriptions are created in constructors

[fjl]

In PR #20390, we attempted to fix an issue where geth crashes if it is stopped before being fully started. The issue is that Subscribe* methods can fail if they use event.SubscriptionScope and the scope is closed before all subscriptions are created. There are two ways to fix this issue:

• Remove uses of SubscriptionScope for internal event providers such as core.BlockChain.
• Ensure internal subscriptions are created in constructor functions before start.

[karalabe]

Just had a PR CI build crash with this :P

https://travis-ci.org/ethereum/go-ethereum/jobs/636825311#L785

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xcb1d4f]
goroutine 5115687 [running]:
github.com/ethereum/go-ethereum/les.(*serverHandler).broadcastHeaders(0xc002215b80)
	/home/travis/gopath/src/github.com/ethereum/go-ethereum/les/server_handler.go:906 +0xcf
created by github.com/ethereum/go-ethereum/les.(*serverHandler).start
	/home/travis/gopath/src/github.com/ethereum/go-ethereum/les/server_handler.go:93 +0x6c
FAIL	github.com/ethereum/go-ethereum/les	15.760s