Reduce DB locking issues in RESTful tests

Add environment variable SERVALD_RHIZOME_DB_RETRY_LIMIT to extend the
Rhizome DB retry period, set it to 60 seconds in RESTful tests to
override the default 5 seconds
Fix RESTful MeshMS newsince ACK logic

/restful/meshms/newsince/SID1/SID2/newsince/TOKEN/messagelist.json now
always lists an ACK as soon as it is received, instead of waiting for a
message to arrive as well.
Fix non-deterministic assertion failure under load