Fix race, don't modify mpost

[janardhan1993]

Clear txnMap before drop all so that future deltas won't be written to
disk.

Fix some tests and blocking fix conflicting transactions

update

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This change is 

[manishrjain]

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Reviewed 15 of 15 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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posting/list.go, line 542 at r1 (raw file):

	midx := 0
	var deleteTs uint64
	if l.markdeleteAll > 0 {

if == 0, else if ..

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worker/mutation.go, line 578 at r1 (raw file):

	}
	commitTimestamps := resp.Ts
	x.AssertTruef(len(startTimestamps) == len(commitTimestamps), "%+v %+v\n", startTimestamps, commitTimestamps)

x.AssertTrue.

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