This adds the function NewEd25519TLSCertPair which uses Ed25519 keys
rather than ECDSA keys as required by NewTLSCertPair.
Conditional compilation is used to provide the function only on Go 1.13.
It also updates the documentation accordingly.
This adds the go 1.11 directive to all of the modules in order to
clearly mark they build and work with that version. Go 1.12 modified
the tools such that tidy will automatically add the new version to
modules that do not already have a directive and that would prevent
builds on Go 1.11 through Go 1.11.3 which is not desirable.
This adds module support for the versioned go toolchain. In particular,
the following packages are defined as modules:
* addrmgr
* blockchain
* certgen
* chaincfg
* connmgr
* database
* dcrjson
* dcrutil
* gcs
* hdkeychain
* mempool
* mining
* peer
* rpcclient
* txscript
* wire
It does not update the travis build environment or README since it is
experimental at this point.
