MB-38327: Interpret access logs written before mad-hatter correctly

An issue was encountered during offline upgrade from 5.0.1 to 6.5.0.
It was found to be while reading the access log f

MB-38327: Interpret access logs written before mad-hatter correctly

An issue was encountered during offline upgrade from 5.0.1 to 6.5.0.
It was found to be while reading the access log for the first time
after the upgrade.

This process requires 6.5.0 to read a older version of the access log
format (containing MutationLogEntryV2 rather than the current, V3).
SerialisedDocKey was changed in 6.5.0 to treat the namespace prefix as
part of the key, rather than as a separate byte.

This required a compensatory change in MutationLogEntryV2 in 6.5.0,
however there was a small bug in how this was done; the struct was
padded by an additional byte, ensuring the key bytes started at the
same offset in the LogEntry, but this _also_ shifted the offset of the
length field.

This lead to 6.5.0 expecting to find the key length field one byte
later than it should - this caused it to read the docNamespace and
treat it as the length. This was always 0 prior to mad-hatter, so the
key was treated as zero length.

However - when "upgrading" the V2 struct to V3 to use it, the code
expected to strip a 1 byte prefix from the key (the docNamespace).
i.e., start + 1, length - 1. This lead to an underflow, and the V3
struct constructor copied the key and read/wrote past the end of
allocated memory.

Fixed by introducing a LegacySerialisedDocKey, which has the previous
layout.

Change-Id: I53e8491fd8afa7927d069904e19b3502ecbdd072
Reviewed-on: http://review.couchbase.org/123930
Well-Formed: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>

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