mesh: Add a define representing max transaction id
mesh: Add a define representing max transaction id

This value was used quite often in the file so adding a
descriptive name should improve readability.

this is port of a40fa45b7417ce1953e99ec36ea0842f3a307544

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble