Add row_ref_equals hook for check_exclusion_or_unique_constraint to w…

…ork with rowid

Rework handling of running transactions by extensions.

 * RunningTransactionsExtension structure as a part of xl_running_xacts and
   RunningTransactionsData to be filled by extensions.
 * getRunningTransactionsExtension hook to fill RunningTransactionsExtension.
 * Add nextXid field to CSNSnapshotData.
 * SnapBuildGetCSNSnaphot() function to modify CSNSnapshotData in the
   SnapBuild.
 * waitSnapshotHook hook to wait for transactions inside the snapshot
   builder.

Rework handling of running transactions by extensions.

 * RunningTransactionsExtension structure as a part of xl_running_xacts and
   RunningTransactionsData to be filled by extensions.
 * getRunningTransactionsExtension hook to fill RunningTransactionsExtension.
 * Add nextXid field to CSNSnapshotData.
 * SnapBuildGetCSNSnaphot() function to modify CSNSnapshotData in the
   SnapBuild.
 * waitSnapshotHook hook to wait for transactions inside the snapshot
   builder.

Rework handling of running transactions by extensions.

 * RunningTransactionsExtension structure as a part of xl_running_xacts and
   RunningTransactionsData to be filled by extensions.
 * getRunningTransactionsExtension hook to fill RunningTransactionsExtension.
 * Add nextXid field to CSNSnapshotData.
 * SnapBuildGetCSNSnaphot() function to modify CSNSnapshotData in the
   SnapBuild.
 * waitSnapshotHook hook to wait for transactions inside the snapshot
   builder.

Rework handling of running transactions by extensions.

 * RunningTransactionsExtension structure as a part of xl_running_xacts and
   RunningTransactionsData to be filled by extensions.
 * getRunningTransactionsExtension hook to fill RunningTransactionsExtension.
 * Add nextXid field to CSNSnapshotData.
 * SnapBuildGetCSNSnaphot() function to modify CSNSnapshotData in the
   SnapBuild.
 * waitSnapshotHook hook to wait for transactions inside the snapshot
   builder.

Fix copying datum in ExecModifyTable()

Save the alignment.

Fix copying datum in ExecModifyTable()

Save the alignment.

fix pg_rewind docs

fix pg_rewind docs

Extend tableam->relation_size() functionality to use different calcul…

…ation methods.

This allows to output size metrics for relations and indexes provided by OrioleDB extension using existing PG functions.

If extension function doesn't support the requested method it should output negative value and thus asking to fall
back to using PG internal calculation e.g.

- Orioledb relation_size outputs -1 for bridged indexes to fallback to counting them as PG indexes
- PG table_block_relation_size outputs -1 for any method except the only DEFAULT_SIZE that it supports.

This API relies on table AM extensibility and doesn't use index AM extensibility yet, which could look more logical,
but also more complicated.