Fix for concurrency race

[thorfour]

If we start a new transaction, reads that started after the last
committed write, but before the compaction wont be able to find those commited
reads. Instead copy the watermark as the tx id to allow those reads to
be able to read all the committed data.

This is should be the fix for #71

[thorfour]

hmm I think this caused a different problem where we now read more data Fixed by #85

--- FAIL: Test_Table_Concurrency (4.62s)
    --- FAIL: Test_Table_Concurrency/1024 (1.10s)
        table_test.go:33: level=error msg="failed to delete granule during split"

        table_test.go:33: level=debug msg="read blocks" n=0

        table_test.go:523:
            	Error Trace:	table_test.go:523
            	Error:      	Not equal:
            	            	expected: 8000
            	            	actual  : 9663
            	Test:       	Test_Table_Concurrency/1024
            	```

[thorfour]

With the latest fix from @ostafen and this fix I'm no longer seeing any failures with the concurrency test.

[MadhavJivrajani]

Not sure if I'm missing something but I tried running the test again with the race detector enabled and I still get races detected:

gh pr checkout 84

HEAD is at a3e6195e7beef1e77eff933d569555eb474772f7

go test -race -run Test_Table_Concurrency

The logs can be found here: https://gist.github.com/MadhavJivrajani/fa69278be76cdf18143dcde533f0e193

Edit: This is probably unrelated to the bug being fixed by the PR tho

[thorfour]

Not sure if I'm missing something but I tried running the test again with the race detector enabled and I still get races detected:

gh pr checkout 84

HEAD is at a3e6195e7beef1e77eff933d569555eb474772f7

go test -race -run Test_Table_Concurrency

The logs can be found here: https://gist.github.com/MadhavJivrajani/fa69278be76cdf18143dcde533f0e193

Edit: This is probably unrelated to the bug being fixed by the PR tho

This is a new one, but definitely unrelated to this fix. Can you open a second issue with this information?

[MadhavJivrajani]

Created #86

[brancz]

This is safe to do because we're not actually adding new data, we're just compacting it, right?

[thorfour]

Correct