Improve export performance
Improve export performance

Benchmark improvements with this change:

benchmark                                   old ns/op     new ns/op     delta
BenchmarkExportTSMFloats_100s_250vps-4      23206480      10279106      -55.71%
BenchmarkExportTSMInts_100s_250vps-4        17995000      5762310       -67.98%
BenchmarkExportTSMBools_100s_250vps-4       17067605      4235467       -75.18%
BenchmarkExportTSMStrings_100s_250vps-4     54846997      34682568      -36.76%
BenchmarkExportWALFloats_100s_250vps-4      23459937      10436297      -55.51%
BenchmarkExportWALInts_100s_250vps-4        18747150      6236062       -66.74%
BenchmarkExportWALBools_100s_250vps-4       17988273      4814358       -73.24%
BenchmarkExportWALStrings_100s_250vps-4     59700802      35815739      -40.01%

benchmark                                   old allocs     new allocs     delta
BenchmarkExportTSMFloats_100s_250vps-4      201442         51738          -74.32%
BenchmarkExportTSMInts_100s_250vps-4        201442         51728          -74.32%
BenchmarkExportTSMBools_100s_250vps-4       201441         51638          -74.37%
BenchmarkExportTSMStrings_100s_250vps-4     404092         201584         -50.11%
BenchmarkExportWALFloats_100s_250vps-4      250322         75627          -69.79%
BenchmarkExportWALInts_100s_250vps-4        250323         75617          -69.79%
BenchmarkExportWALBools_100s_250vps-4       250321         75527          -69.83%
BenchmarkExportWALStrings_100s_250vps-4     452868         225291         -50.25%

benchmark                                   old bytes     new bytes     delta
BenchmarkExportTSMFloats_100s_250vps-4      5170539       2351789       -54.52%
BenchmarkExportTSMInts_100s_250vps-4        5143189       2331276       -54.67%
BenchmarkExportTSMBools_100s_250vps-4       3724951       2143780       -42.45%
BenchmarkExportTSMStrings_100s_250vps-4     17131400      10796281      -36.98%
BenchmarkExportWALFloats_100s_250vps-4      4487868       1468109       -67.29%
BenchmarkExportWALInts_100s_250vps-4        4458395       1452359       -67.42%
BenchmarkExportWALBools_100s_250vps-4       2838719       1258755       -55.66%
BenchmarkExportWALStrings_100s_250vps-4     16787201      10010700      -40.37%

Also, after improving those benchmarks, I did a time-filtered export on
a 450MB TSM file to a 21GB plain text output, with and without the
bufio.BufferedWriter.

Without buffering, it took about 263s, and with buffering, it took about
60s, for a delta of about -77%.

influxdb/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,linearb/influxdb,mark-rushakoff/influxdb,benbjohnson/influxdb,otoolep/influxdb,archsaber/influxdb,pkittenis/influxdb,vladlopes/influxdb,otoolep/influxdb,nooproblem/influxdb,otoolep/influxdb,otoolep/influxdb,mark-rushakoff/influxdb,jgeiger/influxdb,influxdb/influxdb,runner-mei/influxdb,pkittenis/influxdb,influxdata/influxdb,influxdb/influxdb,nooproblem/influxdb,archsaber/influxdb,vladlopes/influxdb,pkittenis/influxdb,linearb/influxdb,chenziliang/influxdb,nooproblem/influxdb,otoolep/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,pkittenis/influxdb,jgeiger/influxdb,benbjohnson/influxdb,mark-rushakoff/influxdb,chenziliang/influxdb,runner-mei/influxdb,jgeiger/influxdb,jgeiger/influxdb,runner-mei/influxdb,archsaber/influxdb,otoolep/influxdb,chenziliang/influxdb,linearb/influxdb,pkittenis/influxdb,pkittenis/influxdb,vladlopes/influxdb,jgeiger/influxdb,influxdb/influxdb,jgeiger/influxdb,archsaber/influxdb,benbjohnson/influxdb,linearb/influxdb,benbjohnson/influxdb,mark-rushakoff/influxdb,runner-mei/influxdb,linearb/influxdb,li-ang/influxdb,chenziliang/influxdb,li-ang/influxdb,influxdb/influxdb,vladlopes/influxdb,nooproblem/influxdb,nooproblem/influxdb