See if disposables are compatible with http package's connection pools

[jackfirth]

If this package lives up to it's ideals, then it should be possible to implement http connection pools by simply wrapping http connections in a disposable, and use generic operations on disposables to implement the pool transparently.

[jackfirth]

This should be possible now by combining the following:

• Disposable pools
• A refresh event to check that connections are still alive before reusing them (see #95)
• Transients to allow explicitly closing connections, even pooled ones (see #92)
• Disposable memoization to group together connections to different hosts (see #96)
• Custodian augmented disposables to ensure connection ports are associated with the correct custodian (see #94)

Only thing missing is automagic timeout logic, which might belong in transient.