Fix mdb_txn_begin(&read-only txn) calls. Test mdb_env_set_maxreaders().
Rename DBI open/close functions. Move mdb_dbi_close() out of txn.

Just use srand/rand, we don't care about the quality
of the random numbers, we just want some data.

Avoid naming conflicts with other mdb* packages. Bump version to 0.9.5

Various mistakes when converting from previous data structures.
Add a few more debug asserts/sanity checks.
Split all "if (foo) return" constructs to separate lines to allow
easier breakpoint setting.

Add mtest6 for checking mdb_split() behavior. This needs to be
expanded to check rebalance/merge cases too.