The getblockchaininfo call in bitcoind uses a commonly used lock,
csmain, in bitcoind. This made the endpoint unsuitable for a health
check, because some nodes were seeing waits up to 5 minutes (!). This
commit updates our health check function to use the uptime api, provided
our bitcoind version is > 0.15, when the api was added. We do not need
to switch our health check for btcd, because it has more granular
locking.
