Reduce meek-client sessionIdLength to 8 from 32, and strip trailing '='.

This reduces the encoded length of a session ID from 44 bytes down to
12. If we have 1000 simultaneous users, and they're all idle and polling
at 5 requests/s, then this change saves 1000*(44-12)/5 = 6.4 KB/s.
According to https://trac.torproject.org/projects/tor/ticket/12778, the
header length is about 413 bytes, so the shorter session ID saves about
7% in the header.