Concurrently allocated sessions get into trouble

[Keith Hodges]

Dear Chris,

I discovered that opening a session, before a prior session had 
completed being opened resulted in two open repositorycontrollers. i.e. 
openedOn: was returning nil for a controller that was already in the 
process of being opened.

This also results in clients being given the same id for multiple sessions.

I solved this by putting a critical block around our #newSession code.

Keith