BCH Buffer Update

[micahriggan]

• new Buffer() is deprecated
  • https://nodejs.org/api/buffer.html#buffer_new_buffer_array
• new Buffer([]) should be Buffer.from([])
• new Buffer(9) should be Buffer.alloc(9)
• BufferWriter should keep track of length as you write
• Transaction.hash getter should cache the last hash, until you change it.

Before Changes

Benchmarking Script
---------------------------------------
isPublicKeyHashIn x 42,112,577 ops/sec ±2.14% (421 runs sampled)
isPublicKeyOut x 30,459,210 ops/sec ±2.80% (430 runs sampled)
toAddress x 65,897 ops/sec ±3.15% (323 runs sampled)
getAddressInfo x 86,997 ops/sec ±2.68% (325 runs sampled)
Done
----------------------------------------------------------------------
Finished

After Changes

Benchmarking Script
---------------------------------------
isPublicKeyHashIn x 65,906,640 ops/sec ±0.98% (516 runs sampled)
isPublicKeyOut x 51,775,114 ops/sec ±0.92% (510 runs sampled)
toAddress x 122,002 ops/sec ±1.03% (437 runs sampled)
getAddressInfo x 143,790 ops/sec ±0.93% (528 runs sampled)
Done
----------------------------------------------------------------------
Finished

[matiu]

There is a failing test at:
https://travis-ci.org/bitpay/bitcore-lib/builds/340632730#L1213

Could you check it?