Fix the sieving of BiTwin candidates (thanks to rdebourbon).

The last member of a BiTwin chain was not sieved at all if the target chain length was odd.
The chains/day estimate was also inflated on mainnet because of this issue.
Changed the default value for 'sieveextensions' to 9 for mainnet.

A value of 9 seems to give slightly higher chains/day after commit 09466ac127.
Remove a redundant check in the sieve (thanks cabin).