Speedup tinytds adapter fetch speed by up to 60%

This code uses the fact that TinyTds::Result#fields returns a
mutable array.  By mutating the members of the fields array before
calling each, we no longer need to manually create the hash, we
can have tiny_tds create the hashes in C using the symbol column
names provided by Sequel.
Don't introspect columns without explicit selects when CTEs are used

CTEs can change the which table is actually being referenced,
so you can no longer rely that a table identifier is representing
the a regular table.
Speedup mysql2 adapter with identifier output method fetch speed by up to 50%

This is a similar change as was made to the tinytds adapter, mutating the
fields of the result before calling each, and having the driver create
the hashes for us.