util: declare wire protocol support of compression engines This patch implements a new compression engine API allowing compression engines to declare support for the wire protocol. Support is declared by returning a compression format string identifier that will be added to payloads to signal the compression type of data that follows and default integer priorities of the engine. Accessor methods have been added to the compression engine manager class to facilitate use. Note that the "none" and "bz2" engines declare wire protocol support but aren't enabled by default due to their priorities being 0. It is essentially free from a coding perspective to support these compression formats, so we do it in case anyone may derive use from it.

--- a/mercurial/help/internals/wireprotocol.txt	Sat Dec 24 13:56:36 2016 -0700
+++ b/mercurial/help/internals/wireprotocol.txt	Sat Dec 24 13:51:12 2016 -0700
@@ -269,6 +269,17 @@
 supported compression formats. The order of the compression formats is in
 server-preferred order, most preferred first.
 
+The identifiers used by the official Mercurial distribution are:
+
+bzip2
+   bzip2
+none
+   uncompressed / raw data
+zlib
+   zlib (no gzip header)
+zstd
+   zstd
+
 This capability was introduced in Mercurial 4.1 (released February 2017).
 
 getbundle