"insert into.. select * from"

When inserting into a partitioned table using 'insert into
<target> select * from <src>', read_buffer_size bytes of memory
are allocated for each partition in the target table.

This resulted in large memory consumption when the number of
partitions are high.

This patch introduces a new method which tries to estimate the
buffer size required for each partition and limits the maximum
buffer size used to maximum of 10 * read_buffer_size, 
11 * read_buffer_size in case of monotonic partition functions.