progress on automerge of segments in levels
bg merge jobs now return the list of guids they created.  added ability to merge an entire level.  added public IDatabase API to get a list of backgroundmergejobs so the caller could wait on them if desired.  removed IDatabase.MergeAll.
notes
code review cleanup
ported seek_ge_le_bigger_multicursor test to f#, complete with simplepagemanager and below-IDatabase access to btree segments in separate files
initial implementation of reusing pages that have been freed.  this revealed some bugs in the btree code involving assumptions about the ordering of pages in a segment.
notes.  and CountPages method on PageBlock struct.