util/tracer: Add DISABLE_TRACER env variable.
util/tracer: Add DISABLE_TRACER env variable.

Tracing every operation has a very noticeable performance impact. This
is a hammer to disble it. We need to either optimize tracing, or make it
sample only a fraction of operations or both.

name                   old time/op  new time/op  delta
Insert1_Cockroach-8     584µs ± 1%   492µs ± 1%  -15.86%    (p=0.000 n=9+7)
Insert10_Cockroach-8    965µs ± 1%   883µs ± 1%   -8.46%   (p=0.000 n=9+10)
Insert100_Cockroach-8  4.61ms ± 1%  4.52ms ± 1%   -1.95%  (p=0.000 n=10+10)
Update1_Cockroach-8    1.37ms ± 1%  1.19ms ± 1%  -13.63%  (p=0.000 n=10+10)
Update10_Cockroach-8   8.48ms ± 1%  6.85ms ± 1%  -19.24%   (p=0.000 n=9+10)
Update100_Cockroach-8  80.4ms ± 1%  64.8ms ± 0%  -19.39%   (p=0.000 n=10+9)
Delete1_Cockroach-8    1.53ms ± 4%  1.51ms ± 1%   -1.85%  (p=0.004 n=10+10)
Delete10_Cockroach-8   2.64ms ± 1%  2.39ms ± 1%   -9.65%   (p=0.000 n=9+10)
Delete100_Cockroach-8  11.9ms ± 3%  11.8ms ± 1%   -1.27%   (p=0.028 n=10+9)
Scan1_Cockroach-8       375µs ± 2%   303µs ± 0%  -19.13%   (p=0.000 n=10+9)
Scan10_Cockroach-8      432µs ± 1%   356µs ± 1%  -17.48%  (p=0.000 n=10+10)
Scan100_Cockroach-8     866µs ± 1%   773µs ± 1%  -10.77%  (p=0.000 n=10+10)

bg451/cockroach,paperstreet/cockroach,nieyy/cockroach,paperstreet/cockroach,arjun4084346/cockroach,nieyy/cockroach,il9ue/cockroach,bg451/cockroach,veteranlu/cockroach,cuongdo/cockroach,il9ue/cockroach,nieyy/cockroach,veteranlu/cockroach,arjun4084346/cockroach,paperstreet/cockroach,glycerine/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,sunya123/cockroach,cockroachdb/cockroach,marcuswestin/cockroach,marcuswestin/cockroach,glycerine/cockroach,maxlang/cockroach,il9ue/cockroach,cockroachdb/cockroach,sunya123/cockroach,petermattis/cockroach,nieyy/cockroach,vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,veteranlu/cockroach,il9ue/cockroach,cuongdo/cockroach,vaibhavarora/cockroach,maxlang/cockroach,marcuswestin/cockroach,cockroachdb/cockroach,nieyy/cockroach,maxlang/cockroach,petermattis/cockroach,maxlang/cockroach,marcuswestin/cockroach,cuongdo/cockroach,nieyy/cockroach,petermattis/cockroach,sunya123/cockroach,maxlang/cockroach,cockroachdb/cockroach,petermattis/cockroach,vaibhavarora/cockroach,sunya123/cockroach,vaibhavarora/cockroach,maxlang/cockroach,vaibhavarora/cockroach,bg451/cockroach,vivekmenezes/cockroach,maxlang/cockroach,il9ue/cockroach,marcuswestin/cockroach,paperstreet/cockroach,bg451/cockroach,sunya123/cockroach,veteranlu/cockroach,bg451/cockroach,cuongdo/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,nieyy/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,petermattis/cockroach,paperstreet/cockroach,paperstreet/cockroach,cuongdo/cockroach,arjun4084346/cockroach,sunya123/cockroach,il9ue/cockroach,arjun4084346/cockroach,il9ue/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,cuongdo/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,il9ue/cockroach,glycerine/cockroach,vivekmenezes/cockroach,petermattis/cockroach,marcuswestin/cockroach,cuongdo/cockroach,cockroachdb/cockroach,sunya123/cockroach,veteranlu/cockroach,glycerine/cockroach,marcuswestin/cockroach,petermattis/cockroach,nieyy/cockroach,vivekmenezes/cockroach,glycerine/cockroach,veteranlu/cockroach,cockroachdb/cockroach,bg451/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,cuongdo/cockroach,petermattis/cockroach,glycerine/cockroach,bg451/cockroach,marcuswestin/cockroach,sunya123/cockroach,vaibhavarora/cockroach,glycerine/cockroach,veteranlu/cockroach