Client block header

[tynes]

• Adds getBlockHeader method to the NodeClient
• Adds header method to bcoin-cli
• Updates tests to use getBlockHeader over get('/header')

Port of bcoin-org/bclient#21

[braydonf]

LGTM

[braydonf]

One thing I noticed when testing, there is an oddity with the behavior for hashes that are not 32 bytes, because of this pattern:

if (hash.length !== 64)
  hash = parseInt(hash, 10);

For example if hash is 3e364f28633de01e6707bb2a4eb78334dbd1f4cb it will become 3, and actually return a block at height 3. This is a pattern that is used several places, so I'll open an issue for it.

[jjggmimi]

Good find