Resolves issue: #1302

Description

This PR batches KVPair updates by key-prefix in to resolve errors for updates with more than 64 KVPairs. KVPairs are batched by key-prefix to ensure keys for a single "cluster" are updated in a single transaction atomically as only 5 x KVPair updates are necessary (< 64)

Details:

• Add ConsulMaxKVsPerTransaction config-param for increasing transaction size, default to 5
  • Default 5 causes 1 x transaction per cluster as there are 5 x KVs each
  • Add logic to ensure the value is between 5 (# of KVs/cluster) and 64 (a Consul API limit)
• Group KVPairs by key-prefix in .DistributePairs()
  • Example mysql/master/cluster1, mysql/master/cluster1/hostname, mysql/master/cluster1/ipv4, mysql/master/cluster1/ipv6 and mysql/master/cluster1/port is grouped into a single transaction at the default ConsulMaxKVsPerTransaction=5
• Concurrently perform grouped KVPair updates with goroutines + sync.WaitGroup
• Testing
  • Add > 64-op transaction error Transaction contains too many operations... to mock Consul server in unit test
  • Add test for .DistributePairs()
• docs/kv.md update

I'm not 100% sure if my changes to go/config/config.go fit with the theme. I also don't like that I made a constant for the number of KVPairs per "cluster". Any suggestions appreciated!

Example debug-logging from .DistributePairs():

2021-02-17 10:25:35 DEBUG consulTxnStore.DistributePairs(): datacenter: dc1; getTxns: 0, setTxns: 0, skipped: 530, existing: 0, written: 0, failed: 0
2021-02-17 10:25:35 DEBUG consulTxnStore.DistributePairs(): datacenter: dc2; getTxns: 0, setTxns: 0, skipped: 530, existing: 0, written: 0, failed: 0
2021-02-17 10:25:35 DEBUG consulTxnStore.DistributePairs(): datacenter: dc3; getTxns: 0, setTxns: 0, skipped: 530, existing: 0, written: 0, failed: 0
2021-02-17 10:25:35 DEBUG consulTxnStore.DistributePairs(): datacenter: dc4; getTxns: 0, setTxns: 0, skipped: 530, existing: 0, written: 0, failed: 0

cc @shlomi-noach