1-key cache

We may consider adding a 1-key cache or similar to improve the performance:

Comment from SOFTHSM-10:

I made a quick and dirty 1-key cache. Just to test the concept (nothing that should be pushed to the general repository). Saving the key handle and private key in the session. The performance in SoftHSMv2+OpenSSL was improved with 34.6% and SoftHSMv2+Botan was improved with a whopping 247.6%.

SoftHSMv2 OpenSSL (1-key cache):
ods-hsmspeed -r SoftHSM -i 50000 -s 1024 -t 4
2281.72 sig/s

SoftHSMv2 Botan (--with-gnump) (1-key cache):
ods-hsmspeed -r SoftHSM -i 50000 -s 1024 -t 4
3698.57 sig/s