fix println performance regression

[amitmurthy]

Addresses #10650

The first time is for writing to a file, the second to STDOUT

$ time julia i2050.jl
real    0m37.606s
user    0m35.112s
sys     0m1.615s

$ time julia i2050_redirect_stdout.jl > juliadump
real    0m39.467s
user    0m37.066s
sys     0m1.787s

The current patch

• locks println only for IO types defining a lock field. Currently only AsyncStream types.
• exports lock, unlock and ReentrantLock. Users writing concurrently to IOStream objects, may lock their writes accordingly. Usage is to be documented.

edit: changed to use stagedfunction
edit2: removed stagedfunction. Only println on AsyncStream is locked.

[JeffBezanson]

I'd rather this not be a staged function. It's overkill, since it will force recompilation for every combination of trailing arguments, whose types aren't even used. Is an isdefined check too slow?

[amitmurthy]

I never realised that! isdefined adds around 1.5 - 2.0 seconds to the results.

[amitmurthy]

Removed the stagedfunction. Only println on AsyncStream objects are locked.

The timings below are on a different system (a MacBook Pro running Ubuntu). The range of timings across multiple runs is quite large on this machine - don't have an explanation for that, but both non-AsyncStream println and AsyncStream println seem to be in the same ballpark.

4 runs of `time julia i2050.jl`

real    0m34.614s
user    0m33.841s
sys     0m1.663s

real    0m45.005s
user    0m41.723s
sys     0m2.425s

real    0m44.310s
user    0m40.538s
sys     0m2.378s

real    0m47.341s
user    0m43.903s
sys     0m2.502s


4 runs of `time julia i2050_redirect_stdout.jl > juliadump`
real    0m37.469s
user    0m36.639s
sys     0m1.678s

real    0m42.083s
user    0m41.228s
sys     0m1.730s

real    0m44.907s
user    0m43.908s
sys     0m1.886s

real    0m46.356s
user    0m45.088s
sys     0m2.153s

[amitmurthy]

Added docs. Will squash and merge in a day or two if there aren't any concerns.