concurrency of opam subcommands

[tuong]

Optimize some "lock" functions so that more operations could be performed at the same time.

Examples :

For instance, if the depending trees of P1 and P2 are disjoint, it could be possible to install P1 & install P2 at the same time.

In the same spirit, one could "init or switch -comp V*" several OCaml at the same time. Then, during a switch from OCaml V1, if we install or remove P in V1, the change will automatically be propagated if the switch has not yet finished.

[samoht]

This has already been partly implemented. opam --switch foo install lwt can happens concurrently with opam --switch bar install lwt