In this commit, we add a new package level mutex. Each time we decode a
new set of chan IDs w/ zlib, we also grab this mutex. The purpose here
is to ensure that we only EVER allocate the maxZlibBufSize globally
across all peers. Otherwise, it may be possible for us to allocate up to
64 MB for _each_ peer, exposing an easy OOM attack vector.
In this commit, we implement zlib encoding and decoding for the channel
range queries. Notably, we utilize an io.LimitedReader to ensure that we
can enforce a hard cap on the total number of bytes we'll ever allocate
in a decoding attempt.
In this commit, we fix a slight bug in the parsing of encoded short
channel ID's. Before this commit, we would always assume that the remote
peer was sending us the sorted+encoded variant of the short channel
ID's. In the case that they weren't (as there isn't yet a feature bit),
we would assert this check and fail early as atm we don't support any
sort of compression.
