Fix race in gc sweep

[dmcgowan]

Removes extra goroutine and calls removal and scan in same thread

[codecov-io]

Codecov Report

Merging #1692 into master will decrease coverage by 0.12%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master   #1692      +/-   ##
=========================================
- Coverage   49.03%   48.9%   -0.13%     
=========================================
  Files          27      27              
  Lines        4087    4071      -16     
=========================================
- Hits         2004    1991      -13     
+ Misses       1664    1662       -2     
+ Partials      419     418       -1

Impacted Files	Coverage Δ
metadata/gc.go	61.96% <100%> (+0.71%)	⬆️
metadata/db.go	66.66% <57.14%> (-0.16%)	⬇️
metadata/content.go	30.62% <0%> (-0.96%)	⬇️

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[dmcgowan]

Rebased on master with race fixes

GC benchmarks with this change

benchmark                                old ns/op     new ns/op     delta
BenchmarkGarbageCollect/10-Sets-8        761321        525422        -30.99%
BenchmarkGarbageCollect/100-Sets-8       6642323       5667259       -14.68%
BenchmarkGarbageCollect/1000-Sets-8      69455007      57987159      -16.51%
BenchmarkGarbageCollect/10000-Sets-8     818302653     733116817     -10.41%

benchmark                                old allocs     new allocs     delta
BenchmarkGarbageCollect/10-Sets-8        2485           2474           -0.44%
BenchmarkGarbageCollect/100-Sets-8       24025          24004          -0.09%
BenchmarkGarbageCollect/1000-Sets-8      258620         258589         -0.01%
BenchmarkGarbageCollect/10000-Sets-8     2805237        2805273        +0.00%

benchmark                                old bytes     new bytes     delta
BenchmarkGarbageCollect/10-Sets-8        144948        133868        -7.64%
BenchmarkGarbageCollect/100-Sets-8       1320973       1228000       -7.04%
BenchmarkGarbageCollect/1000-Sets-8      16509058      14964996      -9.35%
BenchmarkGarbageCollect/10000-Sets-8     174331384     157471128     -9.67%

[crosbymichael]

LGTM

[estesp]

LGTM