Fix bug when import large (>1GB) relations (#2172)

Resolves #2097 

- use timeline modification's `lsn` and timeline's `last_record_lsn` to determine the corresponding LSN to query data in `DatadirModification::get`
- update `test_import_from_pageserver`. Split the test into 2 variants: `small` and `multisegment`. 
  + `small` is the old test
  + `multisegment` is to simulate #2097 by using a larger number of inserted rows to create multiple segment files of a relation. `multisegment` is configured to only run with a `release` build

.github/actions/run-python-test-set/action.yml

@@ -83,6 +83,7 @@ runs:
         # this variable will be embedded in perf test report
         # and is needed to distinguish different environments
         PLATFORM: github-actions-selfhosted
+        BUILD_TYPE: ${{ inputs.build_type }}
         AWS_ACCESS_KEY_ID: ${{ inputs.real_s3_access_key_id }}
         AWS_SECRET_ACCESS_KEY: ${{ inputs.real_s3_secret_access_key }}
       shell: bash -euxo pipefail {0}