Refactor syncing to use streams

[vpulim]

This is a major refactor of the sync components (lib/sync). The reason for the refactor is that there was no support for back-pressure on storing blocks during sync operations. This eventually led to errors when blocks were being downloaded faster than they were being written. This PR replaces the original Fetcher implementation with a new version that uses node Readable/Writable streams which implement back-pressure out of the box. The new stream based Fetcher will queue downloaded blocks and pause until write operations can catch up.

This PR also does a small re-factor of the EthereumService class. Instead of having separate Handlers for handling incoming messages, EthereumService is subclassed into FastEthereumService and LightEthereumService which directly handle incoming messages in a new handle() method.

[coveralls]

Coverage decreased (-1.2%) to 92.731% when pulling e68234c on streams into 8987d88 on master.