mockChainBackend simulates a chain backend and
tracks the number of calls via a private property
called chainCallCount. This commit uses a write
mutex instead of read mutex in the call to GetBlock,
adds a getter method to access chainCallCount
with a read mutex and uses a write mutex in
resetChainCallCount.
This commit was previously split into the following parts to ease
review:
- 2d746f68: replace imports
- 4008f0fd: use ecdsa.Signature
- 849e33d1: remove btcec.S256()
- b8f6ebbd: use v2 library correctly
- fa80bca9: bump go modules
