Upgrade backend to echo v4

[loleg]

Updating the echo framework is a major upgrade from v3 to v4.

See details in #216

[loleg]

Not building due to this issue at the moment during make init validate:

go: downloading golang.org/x/sys v0.13.0
go install github.com/wadey/gocovmerge
no required module provides package github.com/wadey/gocovmerge; to add it:
        go get github.com/wadey/gocovmerge
make: *** [Makefile:84: init] Error 1

[loleg]

Quite sure my issue has to do with this:

proxeus-core/main/www/initial.go

Line 40 in 37991fd

if strings.ToLower(c.Request().Method) == "get" {

[loleg]

All tests passing. I'm doing some additional local validation.

[tafonina]

Looks good, the only thing - be sure that the data is read from env file without manual export as was specified in the build_all.md file