simplify synchronization

[gzliudan]

If we run XDC without any parameter, then XDC should sync with mainnet automatically. So this PR updates the default value of some flags which are related to sync:

flag	old value	new value	status
port	30303		skip
networkid			done by #850
etherbase	"0"	"0x000000000000000000000000000000000000abcd"	done by #855
syncmode	"full"		skip
gcmode	"full"		skip
enable-0x-prefix	true		skip
verbosity	3 (INFO)		skip
datadir	${HOME}/.XDC		skip
XDCx.datadir		datadir/XDCx	deprecated by #854
rpc	false	true	done by #855
rpcaddr	"localhost"	"0.0.0.0"	done by #855
rpcport	8545		skip
rpcapi	""	"eth,debug,net,txpool,personal,web3,XDPoS"	done by #855
rpccorsdomain	""	"*"	done by #855
rpcvhosts	"localhost"	"*"	done by #855
ws	false	true	done by #855
wsaddr	"localhsot"	"0.0.0.0"	done by #855
wsport	8546		skip
wsapi	""	"eth,debug,net,txpool,personal,web3,XDPoS"	done by #855
wsorigins	""	"*"	done by #855
bootnodes			done by #857
gasprice	0.25 gwei	1 wei	done by #855
targetgaslimit	84000000	50000000	done by #855
password	""		skip
ethstats			skip
mine	false		deprecated by #851

[gzliudan]

fixed by PR:

• eth: set networkID to chainId by default #850
• cmd: deprecate the --mine flag #851
• cmd/utils: deprecate flag --XDCx-datadir #854
• cmd/utils: change default value of some flags to simplify sync #855
• params: update bootnodes #857