Go away from the idea of ca_table_insert_row returning offset

The reason for giving up this is the desire to support delayed allocation in an
environment with concurrent writes.  The offset is simply not known until write
time.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
table-backend-log.c: Use ca_file_buffer_* instead of simple writev

This reduces the system call overhead of writing many small (<<1 MB) entries to
a table.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>