v0.21.4

DataFrames v0.21.4

Diff since v0.21.3

Closed issues:

• findfirst/findlast/nextind/prevind not working for eachcol in v0.21.0 (#2229)
• What happened to by()? (#2306)
• Error in showing DataFrame with Distribution column (#2310)

Merged pull requests:

• move lock inside if clause (#2303) (@bkamins)
• update DataFrameRow documentation (#2307) (@bkamins)
• fix show of UnionAll (#2311) (@bkamins)