Optimization: Make ColumnCollection readonly struct

[SergeiPavlov]

According a Dump we have 182,304 instances of this type in memory.
class type overhead over struct is 24 bytes.
Converting it into readonly struct will save >4MB

Also:

• Introduced a new property ColumnCollection.Columns as a replacement of removed inheriting of IReadOnlyList<Column>

[botinko]

@SergeiPavlov just curious, if you checked, that ColumnCollection is not heavely boxed. I this case it can increase allocation rate and make optimization less effective or even make it worse.

[SergeiPavlov]

if you checked, that ColumnCollection is not heavely boxed.

DO never does boxing for such kind of types.
ColumnCollection is a field of RecordSetHeader class and number of instances of these types are almost the same.
That means RecordSetHeader is the main owner of this type