Fix var list in rel node group

[ray6080]

This is a temporary fix for COPY REL table with VAR_LIST properties.

Previously, we were storing VAR_LIST as ku_list entries plus overflow data in separate files. That had been changed for node tables already to a new layout, which consists of offsets and data . Details on the new layout can be found here.

Now as we are moving rel tables to node group based storage, we are also shifting the layout of VAR_LIST on rel tables to the new one. While the problem is that the new layout is not friendly to partially sorted insertions, as we are sorting tuples based on their src node offsets inside the node group.
Take an example of following tuples (dst is ignored):

src	prop
10	[A,B,C]
100	[D]
10	[A,C,D]
100	[E,G,F]

The expected layout is:

offset: [3,6,7,10]
data:   [A,B,C,A,C,D,D,E,G,F]

Existing implementation of VAR_LIST column chunk will always append data to the end of the data column chunk, which would lead to

offset: [3,7,4,10]
data:   [A,B,C,D,A,C,D,E,G,F]

To solve this problem, this PR introduces an AuxVarListColumnChunk, which would keep lists as list_entry_t entires plus data column chunk, and after insertions (during which ColumnChunk::finalize() is called), every list inside it will be appended to a new VarListColumnChunk in a sorted way.