sql: reset values to be bound after execution

Before this patch prepared statements didn't reset bound values after
its execution. As a result, if during next execution cycle not all
parameters were provided, cached values would appear. For instance:

prep = box.prepare('select :a, :b, :c')
prep:execute({{[':a'] = 1}, {[':b'] = 2}, {[':c'] = 3}}
-- [1, 2, 3]
prep:execute({{[':a'] = 1}, {[':b'] = 2}})
-- [1, 2, 3]

However, expected result for the last query should be [1, 2, NULL].
Let's fix it and always reset all binding values before next execution.

Closes #4825