Separate header parsing and block reading

[martindurant]

In order to allow parallel reading by blocks, would prefer to have the class be able to read the header on instantiation, and provide the schema and sync marker. This happens here in fastavro, could copy that.

I appreciate that in cyavro's case, the schema is held in C variables, which we cannot directly serialize. That means, that we need to be able to grab as many bytes as were required for the first part, and pass those around so that a reader with initialized vars can be recreated without having to open and interpret the same file.

The second part will be to be able to read a block of bytes between sync markers. That might need prepending the block with the bytes required to parse the header.

[martindurant]

NB: it should not be necessary to read the first record (and therefore uncompress the first block) for this.