i2p: reuse created I2P sessions if not used

In the case of `i2pacceptincoming=0` we use transient addresses
(destinations) for ourselves for each outbound connection. It may
happen that we
* create the session (and thus our address/destination too)
* fail to connect to the particular peer (e.g. if they are offline)
* dispose the unused session.

This puts unnecessary load on the I2P network because session creation
is not cheap. Is exaggerated if `onlynet=i2p` is used in which case we
will be trying to connect to I2P peers more often.

To help with this, save the created but unused sessions and pick them
later instead of creating new ones.

Alleviates: https://github.com/bitcoin/bitcoin/issues/26754

Github-Pull: #26837
Rebased-From: b906b64eb76643feaede1da5987a0c4d466c581b

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