Refinements of RLN on mainnet spec (#34)

* clarify terminology: epoch length vs period

* clarify parameter update implications for existing memberships

* clarify definition of tx sender as membership keeper

* clarify handling of call chains from EOAs

* clarify slot reuse: from spec-level to impl-level decision

* clarify membership unit price definition

* separate "membership set" and "Merkle tree" as its impl in the spec

* separate withdraw and erase (in line with the implementation)

* clarify functionality table

* edit membership registration section for clarity

* edit for clarity

* clarify types of state transitions in the diagram

* rename "expiration term" to "active state duration"

* clarify requirements: A>0, G >=0

* fix: add semantic break

* make spec less strict on overwriting memberships

* retain remaining grace period time on membership extention

* unify holder and keeper terms - only use holder (fixes #41)

* unify overwrite / reuse terminology

* define period boundaries: start inclusive, end exclusive