Refactor physical off/len indexing of data structs in an object

[jlefevre]

Currently we have 2 types of indexes:

1. physical off/len of a data struct (e.g., flatbuffer or arrow), where each data struct is given a logical sequence number.
2. data content indexes (column values) referring to a data struct's logical sequence number.

These separate logical data from its physical placement. However our build index function currently overloads the building of the physical index when building the content indexes.

Task 1: The physical index building should be separated out of that function, such that it can be called either when building the content indexes, or during a maintenance/reorganization phase, or when appending new data structs to the current object.

[jlefevre]

This issue is subsumed by #50