improvement: reduce times of querying header map

[yangby-cryptape]

I added some log outputs when do tests, I got the follow results:

• Sync chain from height 0 to tip:
  • If always query the header map first.
    • Query the header map first but not found: about tip/2 ~ tip times (mark as x)
    • If not found in the header map, it must be in the store.
  • If query header from store first when the block in that height was already inserted into the chain database.
    • Query the header map first but not found: about tip/4 ~ tip/6 times (mark as y)
    • Query the store first but not found: 0
• The result of my last test: tip = 2_230_953; x = 1_179_787; y = 552_325

[quake]

is it possible to use BlockStatus here?

BlockStatus::UNKNOWN => None,
BlockStatus::STORED => Some(true),
others => Some(false),

[yangby-cryptape]

This assume that tip.number() >= header.number() => BlockStatus::STORED, I'm not sure about that.
I just search store first when tip.number() >= header.number(), but not mean BlockStatus::STORED.

[quake]

bors r=quake,TheWaWaR,driftluo

[bors]

Build succeeded:

• continuous-integration/travis-ci/push