SusiMail: Use input streams for reading mail (ticket #2119)

Rewrite Base64, HeaderLine, and QuotedPrintable decoders
Rewrite and expansion of ReadBuffer class and utilities for streams
Rewrite Mail and MailPart to parse the headers only once
Rewrite MailPart parser
MailPart parser rewrite skips over the data without reading into memory or decoding
MailPart decoder rewrite to decode stream-to-stream
ReadBuffer becomes Buffer interface with multiple implementations
Logging and debugging tweaks