Do not stall Level 0 and 1

[jarifibrahim]

We don't need to stall writes if Level 1 does not have enough space. Level 1 is stored on the disk and it should be okay to have more number of tables (more size) on Level 1 than the max level 1 size. These tables will eventually be compacted to lower levels.

This commit changes the following

• We no longer stall writes if L1 doesn't have enough space.
• We stall L0 only if KeepL0InMemory is true.
• Upper levels (L0, L1, etc) get priority in compaction (previously, level with higher priority score would get preference)

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This change is 

[coveralls]

Coverage increased (+0.3%) to 69.941% when pulling 952173b on ibrahim/remove-l1-stall into 2a90c66 on master.

[martinmr]

but ask Manish for final approval

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ashish-goswami and @manishrjain)

[ashish-goswami]

Please add a test if possible.

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain)

[manishrjain]

Also, do the priority as discussed.

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @jarifibrahim, @manishrjain, and @martinmr)

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levels.go, line 946 at r2 (raw file):

			// computing compactability in order to guarantee progress.
			// Break the loop once L0 has enough space to accommodate new tables.
			if !s.isLevel0Compactable() {

If L0 not in memory, don't stall here.

The only stall should happen based on memory, not disk.

[pawanrawal]

Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @jarifibrahim, and @martinmr)

[manishrjain]

Reviewed 3 of 3 files at r3.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @jarifibrahim)

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levels.go, line 426 at r3 (raw file):

			prios = append(prios, pri)
		}
	}

We used to compact based on the score. But, we decided to compact based on the level, not the priority. So, upper levels always get compacted first, before the lower levels -- this allows us to avoid stalls.

[jarifibrahim]

Travis build failed because of #1197 and #1196 .