This PR introduces a new config field SyncFromCheckpoint for light client.

In some special scenarios, it's required to start synchronization from some
arbitrary checkpoint or even from the scratch. So this PR offers this
flexibility to users so that the synchronization start point can be configured.

There are two relevant configs: SyncFromCheckpoint and Checkpoint.

- If the SyncFromCheckpoint is true, the light client will try to sync from the
  specified checkpoint.

- If the Checkpoint is not configured, then the light client will sync from the
  scratch(from the latest header if the database is not empty)

Additional notes: these two configs are not visible in the CLI flags but only
accessable in the config file.

Example Usage:

[Eth]
SyncFromCheckpoint = true

[Eth.Checkpoint]
SectionIndex = 100
SectionHead = "0xabc"
CHTRoot = "0xabc"
BloomRoot = "0xabc"

PS. Historical checkpoint can be retrieved from the synced full node or light
client via les_getCheckpoint API.

* les, light: remove untrusted header retrieval in ODR

* les: polish

* light: check the hash equality in odr

This commit enables users to specify which signer they want to use while creating their transactOpts.
Previously all contract interactions used the homestead signer. Now a user can specify whether they
want to sign with homestead or EIP155 and specify the chainID which adds another layer of security.

Closes #16484

This change introduces garbage collection for the light client. Historical
chain data is deleted periodically. If you want to disable the GC, use
the --light.nopruning flag.

* les: separate peer into clientPeer and serverPeer

* les: address comments

* les: move the checkpoint oracle into its own package

It's first step of refactor LES package. LES package
basically can be divided into LES client and LES server.
However both sides will use checkpoint package for
status retrieval and verification. So this PR moves
checkpoint oracle into a separate package

* les: address comments

* les: loose restriction of unit tests

* les: update unit tests

* les, light: slim the unit tests

* les: rework clientpool

les: handler separation

* accounts, cmd, contracts, les: integrate clef for transaction signing

* accounts, cmd/checkpoint-admin, signer/core: minor fixups

* all: implement simple checkpoint syncing

cmd, les, node: remove callback mechanism

cmd, node: remove callback definition

les: simplify the registrar

les: expose checkpoint rpc services in the light client

les, light: don't store untrusted receipt

cmd, contracts, les: discard stale checkpoint

cmd, contracts/registrar: loose restriction of registeration

cmd, contracts: add replay-protection

all: off-chain multi-signature contract

params: deploy checkpoint contract for rinkeby

cmd/registrar: add raw signing mode for registrar

cmd/registrar, contracts/registrar, les: fixed messages

* cmd/registrar, contracts/registrar: fix lints

* accounts/abi/bind, les: address comments

* cmd, contracts, les, light, params: minor checkpoint sync cleanups

* cmd, eth, les, light: move checkpoint config to config file

* cmd, eth, les, params: address comments

* eth, les, params: address comments

* cmd: polish up the checkpoint admin CLI

* cmd, contracts, params: deploy new version contract

* cmd/checkpoint-admin: add another flag for clef mode signing

* cmd, contracts, les: rename and regen checkpoint oracle with abigen