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The log file contents are a sequence of 32KB blocks.  The only
exception is that the tail of the file may contain a partial block.

Each block consists of a sequence of records:
   block := record* trailer?
   record :=
	checksum: uint32	// crc32c of type and data[] ; little-endian
	length: uint16		// little-endian
	type: uint8		// One of FULL, FIRST, MIDDLE, LAST
	data: uint8[length]

A record never starts within the last six bytes of a block (since it
won't fit).  Any leftover bytes here form the trailer, which must
consist entirely of zero bytes and must be skipped by readers.

Aside: if exactly seven bytes are left in the current block, and a new
non-zero length record is added, the writer must emit a FIRST record
(which contains zero bytes of user data) to fill up the trailing seven
bytes of the block and then emit all of the user data in subsequent
blocks.

More types may be added in the future.  Some Readers may skip record
types they do not understand, others may report that some data was
skipped.

FULL == 1
FIRST == 2
MIDDLE == 3
LAST == 4

The FULL record contains the contents of an entire user record.

FIRST, MIDDLE, LAST are types used for user records that have been
split into multiple fragments (typically because of block boundaries).
FIRST is the type of the first fragment of a user record, LAST is the
type of the last fragment of a user record, and MID is the type of all
interior fragments of a user record.

Example: consider a sequence of user records:
   A: length 1000
   B: length 97270
   C: length 8000
A will be stored as a FULL record in the first block.

B will be split into three fragments: first fragment occupies the rest
of the first block, second fragment occupies the entirety of the
second block, and the third fragment occupies a prefix of the third
block.  This will leave six bytes free in the third block, which will
be left empty as the trailer.

C will be stored as a FULL record in the fourth block.

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Some benefits over the recordio format:

(1) We do not need any heuristics for resyncing - just go to next
block boundary and scan.  If there is a corruption, skip to the next
block.  As a side-benefit, we do not get confused when part of the
contents of one log file are embedded as a record inside another log
file.

(2) Splitting at approximate boundaries (e.g., for mapreduce) is
simple: find the next block boundary and skip records until we
hit a FULL or FIRST record.

(3) We do not need extra buffering for large records.

Some downsides compared to recordio format:

(1) No packing of tiny records.  This could be fixed by adding a new
record type, so it is a shortcoming of the current implementation,
not necessarily the format.

(2) No compression.  Again, this could be fixed by adding new record types.

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