Add etcd cluster support

#19 - Add etcd cluster endpoints support.

Implemented : https://github.com/etcd-io/etcd/blob/master/Documentation/learning/design-client.md

Exposed lease ttl from etcd client

#18 - Exposed lease ttl from etcd client

Auth Operations Support

1. Add support for Auth operations
2. Inject Headers in all requests
3. Add snapshot support in maintenance calls
4. Add LeaseKeepAlive support
5. Fix rpc exception loop

Bug fixes and enhancements

1. Add overloads for all kv methods to accept appropriate etcd request objects.
2. ResetConnection function now accepts param of type RpcException. Connection will only be reset, in case server is unavailable.

Package Updates

2.1.1

Update packages

Support for new operations

1. Add support for maintenance operations
2. Add support for watching over a range of keys
3. Add GetRange overload which returns RangeResponse
4. Add support for lock operations

Cluster Operations

1. Add support for cluster operations
2. Add support for dotnet core 2.2
3. Update nuget packages

etcd Watch support

Watching over keys is now supported.