Notes on how a multi-table SQL database is mapped into the storage engine.

We assume the storage engine supports key/value pairs where each key and
value is arbitrary binary data.  Keys are in lexicographical order.  In other
words, keys compare according to memcmp() and if one key is a prefix of the
other, the shorter key comes first.  A sample key comparison function is
the following:

   int key_compare(const void *key1, int n1, const void *key2, int n2){
     int c = memcmp(key1, key2, n1<n2 ? n1 : n2);
     if( c==0 ) c = n1 - n2;
     return c;
   }

The key space is logically divided into "tables".  Each key begins with
a varint (in the new lexicographically-ordered unsigned varint format
described separately) that determines the table to which the key belongs.
Table 0 is used for metadata, such as the schema version number.  Table 1
is the SQLITE_MASTER table.  Tables 2 and greater are used for other
tables and indices in the schema.

After the table number, the rest of the key for each entry in a table or
index follows the key encoding documented separately.  The key encoding is
tricky since it must generate keys that sort in order using the key_compare()
function above.  Text is encoded using ucol_getSortKey()-style keys.  The
application-defined collating sequences have to supply a key-generator
function to accommodate this requirement.

The data portion of each entry uses the separately documented data encoding
format.  The data encoding format is similar in concept to the encodings
used for both key and data in SQLite3.  There is a header that contains
data-types followed by the content.  The difference is that the serial
codes are simplified.  And there is a general-purpose extension mechanism.

In the SQLite3 format, both the key and data for an entry were readable.
But in this format, only the data portion is readable because the
ucol_getSortKey()-style encoding for text is not reversible.  Hence
any key content that needs to be accessible must be duplicated using the
data encoding in the data portion of the entry.  This means that index
content is stored twice - once for the key and once for the value.
