* "Bad or missing upstream response" is handled

Block
* switch to condenser when api call does not work on appbase node
* blocknum property returns Null when block is empty
Blockchain
* check improved in  threading blocks
Discussions
* set_next_node_on_empty_reply set to False
Steemnoderpc
* Request Timeout added
Unit tets
* test_account for appbase nodes improved

* stream_threading_performance example added to evaluate threading
* _switch_to_next_node added to Steemnoderpc
* 5 tags restriction removed from post()

move get_similar_account_names from Account to Blockchain + Wallet DRY
Thank you @crokkon for this PR, it looks good to me.

* consolidate / DRY
* update documentation

index.rst: fix blockchain example

Discussions
* Replies_by_last_update and Trending_tags
Blockchain
* get_transaction_hex added
Account
* get_tags_used_by_author added
Docs
* apidifinitions added

beemapi/Nodes
* add option to freeze a node, in order to prevent switching to the next node

ImageUploader can be used to upload an image file or an image in bytes

Scheduled weekly dependency update for week 24

Account
* get_feed_entries, get_blog_authors, get_savings_withdrawals, get_escrow, verify_account_authority, get_expiring_vesting_delegations
* Documentation improved
* Account parameter handling improved
Blockchain
* get_account_reputations added

Blockchain
* thread_limit parameter removed
* batch_mode set to True for threaded blocks()
* block sanity check added for threaded blocks()