Closes #2267
Closes #2561

nextGoodLine() simplified to always return a good guess rather than potentially fail with false. This removes the fail points such as "No good line" and "did not finish exactly".

Only the first sample of 100 lines can now bump the quoteRule. Later sample jumps skip rather than bumping quoteRule because it's more likely that sample jump point was to inside a quoted field. The thread at headPos when reading (which has full lineage from the sof) will bump the quoteRule with certainty then, if needed. This prepares for #2265.

Dirty jump sweeping implemented. Thread at headPos now knows if it finished after nextJumpStart; which will happen if nextJumpStart landed awkwardly inside a quoted field with many newlines causing nextGoodLine to pick a false newline. In this case, jump0 is set to reread the dirty chunk and the team restarted. If too many dirty jumps are encountered, a single thread with full lineage from sof will be needed anyway, so the team is downgraded to single-threaded from that point on. In most cases of a few dirty jumps, they will be swept by the thread at headPos and the remainder of the file continued to be read in parallel.

• simulate bad_fill.csv from fread fails on file with inconsistent # columns #2267. At 35MB it's too big for CRAN but works now.
• simulate issue682.txt from Jump 0 did not finish counting rows exactly where jump 1 found its first good line start #2561. At 2.1MB it's too big for CRAN but works now.
• simulate bad jump point by constructing an embedded field with 5 newlines