[fdcastel]

Addresses item 1 of #74 (feedback).

Adds support for shared snapshots (Firebird 4.0+), so that several transactions, in the same or in different attachments, can read the same database state. For example, a scan split across N connections stays consistent while other transactions write to the table.

Changes

• TransactionOptions::setAtSnapshotNumber(std::uint64_t) / getAtSnapshotNumber(): adds isc_tpb_at_snapshot_number to the TPB. Requires the SNAPSHOT isolation level, and the transaction that owns the snapshot must still be active.
• Transaction::getSnapshotNumber(): reads fb_info_tra_snapshot_number through ITransaction::getInfo(). The info response is parsed the same way as in Blob::getLength(). It throws FbCppException if the server doesn't return the item (Firebird < 4.0).

Tests

TransactionSuite/atSnapshotNumber:

1. A SNAPSHOT transaction on attachment 1 reads its snapshot number.
2. Another transaction then inserts and commits a row.
3. A transaction on a second attachment, started at that snapshot number, reports the same snapshot number and still sees only the first row, the same as the original transaction.
4. A fresh SNAPSHOT transaction gets a larger number and sees both rows.

Full suite: 349 test cases pass (Windows, MSVC, Firebird 5.0.3 server).