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Implementation notes for those who want to modify the code

A. sql buffers
1. Much of the logic is concerned with constructing the SQL to be executed.
   These sql's are stored in one of the global buffers (g_sql_buf).
2. Currently, there are 4 of them (controlled by constant NBUFS in 
   src/sqlite_dataframe.h).
3. It is assumed that R is running on a single thread, so this approach will
   work.
4. Initially, all buffers are 1024 characters long. The buffer sizes are
   stored in the global int array g_sql_buf_sz.
5. When writing strings to a buf with variable length, use 
   _expand_buf([index of buffer], [approx size]). If approx size is greater
   than the buffer's length, it will be reallocated with twice the current
   size.
6. Functions use a naming convention with respect to the buffer it uses.
   Top-level functions may use any of the buffers. Those with name
   ending in a number are expected to use buffers a buffer with that number
   as index, and those "below" it. For example, the function USE_SDF1 may
   use buffers 1 and 2, so those calling it should not store data in those
   buffers that they will need after the function call. Initially I was able
   to make do with 3 buffers (0, 1 & 2), but somewhere I had to add another
   buffer (w/c makes it 4). In most cases, the buffers "below" a number are
   only those buffers up to 2 (e.g., USE_SDF1 uses only buffers 1 & 2) so
   3 can be used by the calling function.

B. USE_SDF1, UNUSE_SDF2
1. Because of the limited