Improve block generation speed
On low configurations like a Raspberry PI, generating a block (the phase before proof) is VERY long.
This is a big handicap, sometimes a dozen of seconds, for these nodes.
It seems that the WoT data is what is long to generate:
2017-08-23T13:38:05+00:00 - debug: It already exists a written certification from 34k6isdkoYZjcEvS3qZqATeTYjY1b3K1gLyW1K77Pea4 which is not chainable yet
2017-08-23T13:38:05+00:00 - debug: It already exists a written certification from DymYJziyjC9pyupKNxT9iukEKrnFSGNCLnxFQtSWJSg5 which is not chainable yet
2017-08-23T13:38:05+00:00 - debug: It already exists a written certification from 5SwfQubSat5SunNafCsunEGTY93nVM4kLSsuprNqQb6S which is not chainable yet
2017-08-23T13:38:05+00:00 - debug: It already exists a written certification from BqegkjBDibg8iUQQnqDrvGLhgcTYTxBnz19LUBNKRdeN which is not chainable yet
2017-08-23T13:38:05+00:00 - debug: It already exists a written certification from 4rWREtAxNS2L427f4vG2LafZNZ9ZLj3cvFFxGyrtFzGL which is not chainable yet
2017-08-23T13:38:06+00:00 - debug: It already exists a written certification from 5SwfQubSat5SunNafCsunEGTY93nVM4kLSsuprNqQb6S which is not chainable yet
2017-08-23T13:38:06+00:00 - debug: It already exists a written certification from GwhyPg6tiESpArzQ2oXXNq7aHPFwBHxp21Eqkf2fvRQV which is not chainable yet
2017-08-23T13:38:06+00:00 - debug: It already exists a written certification from GudqRDfrUYfweHkbscitHiJTBL3UMYejKu3Us4kP3ds9 which is not chainable yet
2017-08-23T13:38:06+00:00 - debug: It already exists a written certification from 78jhpprYkMNF6i5kQPXfkAVBpd2aqcpieNsXTSW4c21f which is not chainable yet
2017-08-23T13:38:06+00:00 - debug: It already exists a written certification from DfAT7wGnRG4c3vnCDSfF5CW8HkwLRL6bMb1ykQPiAgCX which is not chainable yet