This reverts commit 4c2b1bd.
writer: fdatasync => fsync
bplus: update_cb and return_cb should return 0 on fail
test: extended api test with bp_updates
* test: added threaded-rw test * writer: use _XOPEN_SOURCE 600
* writer: define _XOPEN_SOURCE for pread, use fdatasync where possible * refcounter: rename bp__close/bp__open
* compaction is calling bp__init, bp__destroy * split writer, tree creation/destruction in two methods