support reading blocks of all sizes

[emillynge]

Dear Maintainers,
please accept this PR that fixes #229

This issues causes stenographer to panic, if a query is executed under these circumstances:

• Stenotype writes blocks that are not 1Mb
• The query has no criteria or only afterand/or before critieria

Under the above circumstances, the query uses the method BlockFile.AllPackets as a shortcut. This is harcoded to assume that stenotype has written 1MB blocks.

This fix will work, provided that the classes in stenotype extending Packets, ensure that the field block->hdr.bh1.blk_len is populated with the actual size of the block.
This is already the case with PacketsV3 but I do not know whether it is also the case with TestimonyPackets.