Sub filter: eliminate unnecessary buffering. Previously, when a buffer was processed by the sub filter, its final bytes could be buffered by the filter even if they don't match any pattern. This happened because the Boyer-Moore algorithm, employed by the sub filter since b9447fc457b4 (1.9.4), matches the last characters of patterns prior to checking other characters. If the last character is out of scope, initial bytes of a potential match are buffered until the last character is available. Now, after receiving a flush or recycled buffer, the filter performs additional checks to reduce the number of buffered bytes. The potential match is checked against the initial parts of all patterns. Non-matching bytes are not buffered. This improves processing of a chunked response from upstream by sending the entire chunks without buffering unless a partial match is found at the end of a chunk.