send files to contacts (#94)

* schema for sending files

* send file "invitation"

* receive file "invitation"

* send/receive file flow (with stubs)

* update simplexmq

* send and receive the file (WIP - only the first chunk)

* sending and receiving file works (but it is slow)

* use correct terminal output for file sending/receiving

* improve file transfer, support cancellation

* command to show file transfer status and progress

* file transfer tests

* resume file transfer on restart (WIP)

* stabilize test of recipient cancelling file transfer

* trying to improve file transfer on restart

* update SMP block size and file chunk size

* acquire agent lock before chat lock to test whether it avoids deadlock

* fix resuming sending file on client restart

* manual message ACK (prevents losing messages between agent and chat client and stabilizes resuming file reception after restart)

* do NOT send file chunk after receiving it before it is appended to the file

* update file chunk size for SMP block size 8192 (set in smpDefaultConfig)

* save received files to ~/Downloads folder by default; create empty file when file is accepted

* keep file handle used to create empty file

* check message integrity

* fix trying to resume sending file when it was not yet accepted

* fix subscribing to pending connections on start

* update simplexmq (fixes smp-server syntax parser)

src/Simplex/Chat/protocol.md

@@ -65,6 +65,7 @@ refMsgHash = 16*16(OCTET) ; SHA256 of agent message body
 ' x.grp.mem.inv 23456,234 x.text:NNN <invitation> '
 ' x.grp.mem.req 23456,123 x.json:NNN {...} '
 ' x.grp.mem.direct.inv 23456,234 x.text:NNN <invitation>  '
+' x.file name,size x.text:NNN <invitation> '
 ```
 
 ### Group protocol