Optimize XidtoUID map used by live and bulk loader

[manishrjain]

I've spent the last few days looking at how to optimize the live mutation path in Dgraph server. While trying many things in the server (past commits included), I realized my optimizations in the server are not improving things much, the throughput saturating at 20-30K NQuads/sec.

Turns out, it was the live loader which was causing the saturation. The XID to UID assigner was the bottleneck causing the throughput to stagnate, despite the server being underutilized.

This PR fixes that by optimizing the assigner. In particular, I've removed the slow LRU cache. Added buffer to newRanges channel to ensure we always have a range handy when we run out. Made passing badger DB instance optional, so we can avoid doing disk writes if not required. And made other optimizations around how we lock, etc. I also added benchmarks for the assigner, which shows each allocation (tested via parallel benchmark) takes 350 ns/op on my desktop.

With these changes, the live loader throughput jumps to 100K-120K NQuads/sec on my desktop. In particular, pre-assigning UIDs to the RDF/JSON file yields maximum throughput. I can load 140M friend graph RDFs in 25 mins.

Helps with #2975 .

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This change is 

[martinmr]

Reviewed 8 of 8 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain)

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xidmap/xidmap.go, line 60 at r1 (raw file):

}

// This must already have a write lock.

Change comment format to match what the linter expects. Something like:

assign assumes the lock is already acquired at call time.

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xidmap/xidmap.go, line 198 at r1 (raw file):

// BumpTo can be used to make Zero allocate UIDs up to this given number. Attempts are made to
// ensure all future allocations of UIDs be higher than this one, but result is not guaranteed.

nit: results are not guaranteed

[manishrjain]

Reviewable status: 7 of 8 files reviewed, 1 unresolved discussion (waiting on @martinmr)

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xidmap/xidmap.go, line 60 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

Change comment format to match what the linter expects. Something like:

assign assumes the lock is already acquired at call time.

Done.