The SignalTable is used to create watermarks in the wal, and check whether the read item is a watermark It is assumed that the low_watermark will be placed before reading data from the table, and the high_watermark after reading data from the table When reading the wal, we have 2 types of watermarks needed to check in wal (low and high): - low - means that we are now just before the moment, when we start reading from the database - high - means that we have completed reading Also we have 2 types needed to resolve additional problem (success watermark and bad watermark): - success - needed to save the last successfully transferred chunk - bad - it is necessary to mark an invalid watermark type.