fixes #249

added checkpoint duration to engine status #219

    old time new time
10k    20     12
20k    34     23
40k   192     45
80k  1388     71

This reverts commit 3e0d9e32f8669e66ae3087616bba3e86bbb3bc39.

initialize it for full gc if we think it is worth it. This allows for
full gc to run when bringing a basement node up to date.

there may be an abort message waiting to be applied.

in a different order than they began, and blow up a bad assert.

separate the difference between the oldest referenced xid we can use for
implicit promotion vs simple garbage collection.

run full garbage collection when a leafentry has > 1 committed entry.

can iterate over and clean up, which prevents a memory leak when the
fractal tree shared library unloads before all thread-local destructors run

This reverts commit 843f0050.

Should have been on a branch.

This reverts commit 7fa771e0.

Should have been on a branch.

This reverts commit fbd96859.

Should have been on a branch.

This reverts commit d467d3cf.

Should have been on a branch.

the last point we may read the node variable safely.