Fix jore3-importer logic related to validity period inconsistencies b…

…etween

transformed lines and routes.

In the Jore3 data model, the line itself (the `network_lines` table in the
internal database of jore3-importer) has no validity period. Instead, the
line-specific headers (`network_line_headers` table) have validity periods that
do not overlap. Originally, it was thought that a separate line instance would
be created for each line header into the Jore4 database. However, it appears
that Jore3 route directions (`network_route_directions` table) may overlap with
multiple line headers. This causes a transformation problem, because in Jore4
the validity period of a route MUST NOT extend further into the past or future
than the line to which the route belongs.

Basically, there are two options to solve the problem:
(1) For each line in Jore3 (and all related line headers), create only single
    line in Jore4.
(2) Group and join line headers together by validity periods in such a way that
    no route overlaps multiple line instances transformed to Jore4.

In this commit, we have chosen the option (2) to import as much (version) data
as possible into Jore4. In the line export SQL, we group the line headers into
temporally continuous clusters. The last line header of each cluster is given
a Jore4 ID that can be referenced by routes.

Add Jore4 ID to the `network_line_headers` (and the corresponding history) table.
Remove Jore4 ID from the `network_lines` (and corresponding history) table.

Resolves HSLdevcom/jore4#1659