LES Part 3: Event handlers and handling responses

[rphmeier]

Enable the light server with the --serve-light flag. More specific configuration of buffer flow parameters and such will be in the next PR.

Up next:

• request timeouts (currently they'll just pend forever)
• CHT on the server side (how much of this do we want to support?)
• ensure compatibility with geth's LES. Right now there are some messages formatted differently and I'll need to spend quite a bit of time checking and double-checking request formatting. best to get this in review first
• light-sync implementation

To help review:

• EventContext is similar to the NetSyncIo trait in the sync module, only that it's relevant to a higher level set of events than a basic NetworkProtocolHandler.
• I discovered that the buffer flow arguments are actually optional in the handshake pretty late on, although the spec is a bit ambiguous on that point. So handshake parsing has gotten a bit more lenient
• There's a bit of cargo-feature shuffling and I think the IPC code I've written will work but IPC build of ethcore crate is still broken and all my code lives above it for now. These parts of the PR aren't really that important

[coveralls]

Changes Unknown when pulling 7eeac23 on lightserv into ** on master**.

[coveralls]

Coverage decreased (-0.004%) to 85.898% when pulling 7eeac23 on lightserv into f9a24f3 on master.

[coveralls]

Changes Unknown when pulling 503b126 on lightserv into ** on master**.

[coveralls]

Coverage decreased (-0.5%) to 85.589% when pulling 503b126 on lightserv into 2226324 on master.

[rphmeier]

(travis failure unrelated, tests passed before merge and now locally)