mdb_env_close obliterates unrelated readers

[dw]

erp, seems mdb_env_close() with MDB_NOTLS isn't quite ideal yet

since readers aren't tracked in MDB_env, it just blasts everything in the reader table with the same PID

there shouldn't be any occasion where the same process has the environment opened twice, but it seems there is also no way to detect and prevent this condition..

without opening and closing the environment in order to call _readers() (the close, of course, will obliterate the already existing environment even if we manage to
throw an exception)

not sure if it is necessarily a bug. maybe mdb_env_open() should grow a new MDB_ALREADY_OPEN or something error

i can completely work around it in the binding by fstat() and compare inodes with internal list of existing envs prior to mdb_env_open()