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)

doc/todo/smudge.mdwn

@@ -32,14 +32,6 @@ git-annex should use smudge/clean filters.
   when pushing changes committed in such a repo. Ideally, should avoid
   committing implicit unlocks, or should prevent such commits leaking out
   in pushes.
-* Optimisation: See if the database schema can be improved to speed things
-  up. Are there enough indexes? getAssociatedKey in particular does a
-  reverse lookup and might benefit from an index.
-* Optimisation: Reads from the Keys database avoid doing anything if the
-  database doesn't exist. This makes v5 repos, or v6 with all locked files
-  faster. However, if a v6 repo unlocks and then re-locks a file, its
-  database will exist, and so this optimisation will no longer apply.
-  Could try to detect when the database is empty, and remove it or avoid reads.
 
 * Eventually (but not yet), make v6 the default for new repositories.
   Note that the assistant forces repos into direct mode; that will need to