Initial changes for async support

[AdrianCX]

Splitting #163 into separate parts.

Also rebased since @raoulstrackx already helped with some pieces of the PR above.

Code does the following:

• Minor update to comments on Pk (revisited the latest code to check if any issues, none found)
• verify with sig len 0 makes mbedtls assume default siglen and potentially overflow instead of stopping.
• EntropyHolder can have different sources, either shared or not (request from another older PR)
• Ported ALPN changes from @mzohreva branch - NullTerminatedList
• Split off HandshakeContext and made it a sort of base class to Context so we can use it safely. (lots of ideas from @jethrogb here)
• Made Context < T > as opposed to using Any, makes life easier for async and a lot of other use cases.

[jethrogb]

verify with sig len 0 makes mbedtls assume default siglen and potentially overflow instead of stopping.

This is fixed in MbedTLS 2.28, I think?

[jethrogb]

Nit: doesn't need to be Vec.

[jethrogb]

We discussed not using a macro here

[jethrogb]

bors r+

[bors]

Build succeeded:

• continuous-integration/travis-ci/push