Store the address of the MDB_env structure that owns a reader in addition to
its PID and TID, to allow multiple MDB_envs to be open on the same thread,
since on mdb_env_close(), LMDB unconditionally obliterates any readers with a
matching PID. This patch extends the test to (PID, MDB_env*).

diff --git a/lib/mdb.c b/lib/mdb.c
index fd0a3b5..f2ebdfa 100644
--- a/lib/mdb.c
+++ b/lib/mdb.c
@@ -536,6 +536,8 @@ typedef struct MDB_rxbody {
 	txnid_t		mrb_txnid;
 	/** The process ID of the process owning this reader txn. */
 	MDB_PID_T	mrb_pid;
+	/** MDB_env within the process owning this reader txn. */
+	void *      mrb_env;
 	/** The thread ID of the thread owning this txn. */
 	pthread_t	mrb_tid;
 } MDB_rxbody;
@@ -547,6 +549,7 @@ typedef struct MDB_reader {
 		/** shorthand for mrb_txnid */
 #define	mr_txnid	mru.mrx.mrb_txnid
 #define	mr_pid	mru.mrx.mrb_pid
+#define	mr_env	mru.mrx.mrb_env
 #define	mr_tid	mru.mrx.mrb_tid
 		/** cache line alignment */
 		char pad[(sizeof(MDB_rxbody)+CACHELINE-1) & ~(CACHELINE-1)];
@@ -2285,6 +2288,7 @@ mdb_txn_renew0(MDB_txn *txn)
 					return MDB_READERS_FULL;
 				}
 				ti->mti_readers[i].mr_pid = pid;
+				ti->mti_readers[i].mr_env = env;
 				ti->mti_readers[i].mr_tid = tid;
 				if (i == nr)
 					ti->mti_numreaders = ++nr;
@@ -4254,7 +4258,8 @@ mdb_env_close0(MDB_env *env, int excl)
 		 * me_txkey with its destructor must be disabled first.
 		 */
 		for (i = env->me_numreaders; --i >= 0; )
-			if (env->me_txns->mti_readers[i].mr_pid == pid)
+			if (env->me_txns->mti_readers[i].mr_pid == pid
+				&& env->me_txns->mti_readers[i].mr_env == env)
 				env->me_txns->mti_readers[i].mr_pid = 0;
 #ifdef _WIN32
 		if (env->me_rmutex) {