Avoid unnecessary copying of binaries in diameter_tcp

Since messages are accumulated by appending binaries as of three
commits back, the accumulated binary is prone to being copied, as
discussed in the Efficiency Guide. Matching the Message Length header
as bytes are being accumulated is one cause of this, so work around it
by splitting the binary and extracting the length without a match.

This doesn't feel like something that should be necessary: that matching
a binary would cause an append to copy isn't obvious. The first attempt
at simplifying the accumulation was simply to append an incoming binary
to the current fragment, match against <<_, Len:24, _/binary>> to
extract the length, and then test if there are enough bytes for a
message. This lead to horrible performance (response times for 2 MB
messages approximately 100 times worse than previously), and it wasn't
at all obvious that the reason was the accumulated binary being copied
with each append as a result of the match. Using split_binary avoids the
match context that forces the copying.

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