Fix memory leak in live loader

[ashish-goswami]

This PR aims to fix a memory leak in live loader. Currently while getting uid for a NQuad
subject/uid object, we call xidMap.AssignUid(). This subject/uid object is of string type.
We suspect that these uids(strings) are still holding references to original string, which
we get after reading single lines in checker.Parse(). Hence GC is unable to clean these strings.
In this PR, while getting uid for any NQuad subject/uid object, we pass a copy of it to
xidMap.AssignUid().

Original profile:

File: dgraph
Build ID: b80d1c50901c66e79b21afb1ed72a92bed950dc9
Type: inuse_space
Time: May 19, 2020 at 3:52pm (IST)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 62468.74MB, 100% of 62486.76MB total
Dropped 28 nodes (cum <= 312.43MB)
      flat  flat%   sum%        cum   cum%
42551.37MB 68.10% 68.10% 42551.37MB 68.10%  github.com/dgraph-io/dgraph/xidmap.(*XidMap).AssignUid
19902.91MB 31.85% 99.95% 19902.91MB 31.85%  bytes.(*Buffer).ReadString
   13.50MB 0.022%   100% 19923.42MB 31.88%  github.com/dgraph-io/dgraph/chunker.(*rdfChunker).Parse
    0.95MB 0.0015%   100% 42554.34MB 68.10%  github.com/dgraph-io/dgraph/dgraph/cmd/live.(*loader).processLoadFile.func1
         0     0%   100% 19931.42MB 31.90%  github.com/dgraph-io/dgraph/dgraph/cmd/live.(*loader).processFile
         0     0%   100% 19931.42MB 31.90%  github.com/dgraph-io/dgraph/dgraph/cmd/live.(*loader).processLoadFile
         0     0%   100% 42553.38MB 68.10%  github.com/dgraph-io/dgraph/dgraph/cmd/live.(*loader).uid
         0     0%   100% 19931.42MB 31.90%  github.com/dgraph-io/dgraph/dgraph/cmd/live.run.func2

This PR:

File: dgraph
Build ID: d49e85006a57b932fa73218470d0e0b97fbb81d8
Type: inuse_space
Time: May 19, 2020 at 7:33pm (IST)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 28967.65MB, 99.43% of 29134.97MB total
Dropped 71 nodes (cum <= 145.67MB)
      flat  flat%   sum%        cum   cum%
16028.87MB 55.02% 55.02% 16034.87MB 55.04%  github.com/dgraph-io/dgraph/xidmap.(*XidMap).AssignUid
12928.89MB 44.38% 99.39% 12928.89MB 44.38%  strings.(*Builder).WriteString
    8.88MB  0.03% 99.42% 28990.15MB 99.50%  github.com/dgraph-io/dgraph/dgraph/cmd/live.(*loader).processLoadFile.func1
       1MB 0.0034% 99.43% 28975.27MB 99.45%  github.com/dgraph-io/dgraph/dgraph/cmd/live.(*loader).uid

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[manishrjain]

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