fix(wal): fix WAL apply lag calculation

[ideoma]

Fixes bug found by CI in one of the WAL Fuzz tests.

java.lang.RuntimeException: java.lang.AssertionError
	at io.questdb.cairo.TableWriter.processWalBlock(TableWriter.java:1498)
	at io.questdb.cairo.TableWriter.processWalData(TableWriter.java:1592)
	at io.questdb.cairo.wal.ApplyWal2TableJob.processWalCommit(ApplyWal2TableJob.java:467)
	at io.questdb.cairo.wal.ApplyWal2TableJob.applyOutstandingWalTransactions(ApplyWal2TableJob.java:392)
	at io.questdb.cairo.wal.ApplyWal2TableJob.applyWAL(ApplyWal2TableJob.java:119)
	at io.questdb.cairo.wal.ApplyWal2TableJob.doRun(ApplyWal2TableJob.java:573)
	at io.questdb.mp.AbstractQueueConsumerJob.run(AbstractQueueConsumerJob.java:41)
	at io.questdb.mp.Job.run(Job.java:57)
	at io.questdb.griffin.wal.WalWriterFuzzTest.runApplyThread(WalWriterFuzzTest.java:566)
	at io.questdb.griffin.wal.WalWriterFuzzTest.lambda$applyManyWalParallel$1(WalWriterFuzzTest.java:360)

The reason for assertion failure is the negative WAL Lag value. This is the result of incorrect WAL calculation formula.

[ideoma]

[PR Coverage check]

😍 pass : 10 / 10 (100.00%)

file detail

	path	covered line	new line	coverage
🔵	io/questdb/cairo/TableWriter.java	10	10	100.00%