Describe Uncompressed And Compressed Public Keys

A quick little subsection the two different public key formats possible
in Bitcoin.
Add Short Subsection About The Memory Pool
Corrections To Memory Pool Text

Based on feedback from @luke-jr.  (Thanks!)

* Use "may" to indicate that full peers don't need to track unconfirmed txes.

* Change the example given for why non-mining peers might want to track
  unconfirmed txes.

* Note that txes also get dropped from mempool when memory needs
  to be freed.

* Remove paragraph about storing non-relay/non-mine txes.
Describe What Happens To Mempool During A Reorg
Updates To Compressed Keys Based On Reviewer Feedback

Several reviewers kindly provided feedback via IRC tonight, resulting in
the following changes:

* Mention that compressed keys are used in the offical secp256k1
  documentation with the prefix bytes used by Bitcoin. I could not find
  a publicly-available officialish document defining the prefix bytes,
  but they're used repeatedly in the offical SECG documentation we
  autoxref "secp256k1" to, so I went with that.

* Remove the mention that Bitcoin Core uses OpenSSL. While true, there
  is an effort to slowly move away from using OpenSSL for EC in
  Bitcoin Core.

* Change the phrasing of the relative curve location for 0x02 & 0x03 to
  a form more correct for actuality instead of the illustrated
  abstraction.

* Drop quotes around "uncompressed" since that's the term used in the
  official secp256k1 docs, not something Bitcoin-specific.
Merge branches 'compressed-keys' and 'memorypool' into master