MB-16181: Move leb128 decoder function to be after the encoder
MB-16181: Move leb128 decoder function to be after the encoder

A future patch needs to use the unsigned_leb128<T>::getMaxSize
function, so moving the code so we can call it from the decoder.

Change-Id: I97e3e35faf50f4ba997fd91e6e31e913a1192dd0
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/130885
Reviewed-by: Richard de Mellow <3b8534f7ae49c77465cd6f1b512f27ec39f3eddd@couchbase.com>
Tested-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine