add shared guard for setkey

[ben-schwen]

Closes #5230

[github-actions]

• HEAD=setkey_shallow_copy much slower for as.data.table.array improved in #7019
  

Generated via commit 1d17aaa

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	6 minutes and 27 seconds
Installing different package versions	12 minutes and 13 seconds
Running and plotting the test cases	5 minutes and 23 seconds

[aitap]

Part of the problem here is that it's too easy for data.table columns to accumulate a large reference count due to we use shallow copies:

x <- as.data.table(copy(mtcars))
.Internal(inspect(x, 1))
# @55bf23a15f20 19 VECSXP g0c7 [OBJ,REF(1),gp=0x20,ATT] (len=11, tl=1035)
#  @55bf262bdbf0 14 REALSXP g0c7 [REF(4)] (len=32, tl=0) 21,21,22.8,21.4,18.7,...
#   @55bf23cc0f60 14 REALSXP g0c7 [REF(4)] (len=32, tl=0) 6,6,4,6,8,...
#   @55bf23cc17d0 14 REALSXP g0c7 [REF(4)] (len=32, tl=0) 160,160,108,258,360,...
#   @55bf23d49d40 14 REALSXP g0c7 [REF(4)] (len=32, tl=0) 110,110,93,110,175,...
#   @55bf239f09a0 14 REALSXP g0c7 [REF(4)] (len=32, tl=0) 3.9,3.9,3.85,3.08,3.15,...
x[,mean(mpg),by=.(gear,carb)]
.Internal(inspect(x, 1))
# @55bf23a15f20 19 VECSXP g0c7 [OBJ,REF(3),gp=0x20,ATT] (len=11, tl=1035)
#   @55bf262bdbf0 14 REALSXP g0c7 [REF(6)] (len=32, tl=0) 21,21,22.8,21.4,18.7,...
#   @55bf23cc0f60 14 REALSXP g0c7 [REF(5)] (len=32, tl=0) 6,6,4,6,8,...
#   @55bf23cc17d0 14 REALSXP g0c7 [REF(5)] (len=32, tl=0) 160,160,108,258,360,...
#   @55bf23d49d40 14 REALSXP g0c7 [REF(5)] (len=32, tl=0) 110,110,93,110,175,...
#   @55bf239f09a0 14 REALSXP g0c7 [REF(5)] (len=32, tl=0) 3.9,3.9,3.85,3.08,3.15,...

So far, the guidance from above was to clean up our temporary shallow copies by hand in order to lower the reference counts on the columns.