Checkpoint.

- Fix a couple of edge cases while switching between collections.
  - Raise error if attempting to switch collections mid-transaction.
  - Add logic for re-entrancy with long-running transactions.
  - Revert to the prior collection if passing a block to #collection.
  - Add a predicate to tell if you're currently within a long-running transaction.
  - Add separate commit/rollback to long-running transactions.

FossilOrigin-Name: 711239e6fc2f25479a26fb54805da1e5db792f97f28a8b5724e0b38eb11cdb07

spec/mdbx/database_spec.rb

@@ -1,4 +1,5 @@
 #!/usr/bin/env rspec -cfd
+# vim: set nosta noet ts=4 sw=4 ft=ruby:
 
 require_relative '../lib/helper'
 
@@ -113,6 +114,12 @@ RSpec.describe( MDBX::Database ) do
 			expect( db['key'] ).to be_truthy
 		end
 
+		it "revert back to the previous collection when used in a block" do
+			expect( db.collection ).to be_nil
+			db.collection( 'bucket' ) { 'no-op' }
+			expect( db.collection ).to be_nil
+		end
+
 		it "can be cleared of contents" do
 			db.collection( 'bucket' )
 			10.times {|i| db[i] = true }