Eliminate blocking reads in the HTTP2 matcher.
Eliminate blocking reads in the HTTP2 matcher.

The HTTP2 matcher uses io.ReadFull to read the client preface.
If the client sends a string shorter than the preface (e.g.,
SSL version) io.ReadFull will block.

Replace io.ReadFull with Read and assume partial reads will not
match

Fixes #44

soheilhy/cmux