add database benchmark

The benchmark shows that the database access is quite fast indeed!
And, it scales linearly to the number of keys, with one exception,
getAssociatedKey.

Based on this benchmark, I don't think I need worry about optimising
for cases where all files are locked and the database is mostly empty.
In those cases, database access will be misses, and according to this
benchmark, should add only 50 milliseconds to runtime.

(NB: There may be some overhead to getting the database opened and locking
the handle that this benchmark doesn't see.)

joey@darkstar:~/src/git-annex>./git-annex benchmark
setting up database with 1000
setting up database with 10000
benchmarking keys database/getAssociatedFiles from 1000 (hit)
time                 62.77 μs   (62.70 μs .. 62.85 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 62.81 μs   (62.76 μs .. 62.88 μs)
std dev              201.6 ns   (157.5 ns .. 259.5 ns)

benchmarking keys database/getAssociatedFiles from 1000 (miss)
time                 50.02 μs   (49.97 μs .. 50.07 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 50.09 μs   (50.04 μs .. 50.17 μs)
std dev              206.7 ns   (133.8 ns .. 295.3 ns)

benchmarking keys database/getAssociatedKey from 1000 (hit)
time                 211.2 μs   (210.5 μs .. 212.3 μs)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 211.0 μs   (210.7 μs .. 212.0 μs)
std dev              1.685 μs   (334.4 ns .. 3.517 μs)

benchmarking keys database/getAssociatedKey from 1000 (miss)
time                 173.5 μs   (172.7 μs .. 174.2 μs)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 173.7 μs   (173.0 μs .. 175.5 μs)
std dev              3.833 μs   (1.858 μs .. 6.617 μs)
variance introduced by outliers: 16% (moderately inflated)

benchmarking keys database/getAssociatedFiles from 10000 (hit)
time                 64.01 μs   (63.84 μs .. 64.18 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 64.85 μs   (64.34 μs .. 66.02 μs)
std dev              2.433 μs   (547.6 ns .. 4.652 μs)
variance introduced by outliers: 40% (moderately inflated)

benchmarking keys database/getAssociatedFiles from 10000 (miss)
time                 50.33 μs   (50.28 μs .. 50.39 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 50.32 μs   (50.26 μs .. 50.38 μs)
std dev              202.7 ns   (167.6 ns .. 252.0 ns)

benchmarking keys database/getAssociatedKey from 10000 (hit)
time                 1.142 ms   (1.139 ms .. 1.146 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 1.142 ms   (1.140 ms .. 1.144 ms)
std dev              7.142 μs   (4.994 μs .. 10.98 μs)

benchmarking keys database/getAssociatedKey from 10000 (miss)
time                 1.094 ms   (1.092 ms .. 1.096 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 1.095 ms   (1.095 ms .. 1.097 ms)
std dev              4.277 μs   (2.591 μs .. 7.228 μs)

Common.hs

@@ -5,7 +5,7 @@ module Common (module X) where
 import Control.Monad as X
 import Control.Monad.IfElse as X
 import Control.Applicative as X
-import "mtl" Control.Monad.State.Strict as X (liftIO)
+import Control.Monad.IO.Class as X (liftIO)
 
 import Data.Maybe as X
 import Data.List as X hiding (head, tail, init, last)